Keep buffer base & size mask in high registers.

The high registers (r8-r14) cannot be used directly by most of the
instructions in the Cortex-M0 instruction set.

One of the few instructions that can use them is mov, which can use
any pair of registers.

This allows saving two cycles, by replacing two loads (2 cycles each)
with moves (1 cycle each), after stashing the required values in high
registers at startup.

firmware/hackrf_usb/sgpio_m0.s

@@ -66,8 +66,8 @@ shadow registers.
 
 There are two key code paths, with the following worst-case timings:
 
-RX:             151 cycles
-TX:             136 cycles
+RX:             149 cycles
+TX:             134 cycles
 
 Design
 ======
@@ -120,6 +120,8 @@ registers and fixed memory addresses.
 
 /* Allocations of single-use registers */
 
+buf_base          .req r12
+buf_mask          .req r11
 sgpio_data        .req r7
 sgpio_int         .req r6
 buf_ptr           .req r5
@@ -134,6 +136,10 @@ main:
 	// Initialise registers used for constant values.
 	ldr sgpio_int, =SGPIO_EXCHANGE_INTERRUPT_BASE                                           // 2
 	ldr sgpio_data, =SGPIO_SHADOW_REGISTERS_BASE                                            // 2
+	ldr r0, =TARGET_DATA_BUFFER                                                             // 2
+	mov buf_base, r0                                                                        // 1
+	ldr r0, =TARGET_BUFFER_MASK                                                             // 2
+	mov buf_mask, r0                                                                        // 1
 loop:
 	// The worst case timing is assumed to occur when reading the interrupt
 	// status register *just* misses the flag being set - so we include the
@@ -164,7 +170,7 @@ loop:
 	str r1, [sgpio_int, #INT_CLEAR]                                                         // 8
 
 	// ... and grab the address of the buffer segment we want to write to / read from.
-	ldr r0, =TARGET_DATA_BUFFER       // r0 = &buffer                                       // 2
+	mov r0, buf_base                  // r0 = &buffer                                       // 1
 	ldr r3, =TARGET_BUFFER_POSITION   // r3 = &position_in_buffer                           // 2
 	ldr r2, [r3]                      // r2 = position_in_buffer                            // 2
 	add buf_ptr, r0, r2               // buf_ptr = &buffer + position_in_buffer             // 1
@@ -212,7 +218,7 @@ direction_rx:
 done:
 
 	// Finally, update the buffer location...
-	ldr r0, =TARGET_BUFFER_MASK                                                             // 2
+	mov r0, buf_mask                                                                        // 1
 	and r0, buf_ptr, r0    // r0 = (pos_in_buffer + size_copied) % buffer_size              // 1
 
 	// ... restore &position_in_buffer, and store the new position there...