Fix SPM HAL test

When accessing non-secure ram and flash r1 was actually used by the calling function.
Change to a callee saved register.

TESTS/mbed_hal/spm/main.cpp

@@ -84,7 +84,7 @@ __attribute__((naked)) void call_mem(uint32_t addr)
     // since exception will be generated for invalid memory access.
     // Other instructions are for calling do_nothing function according to AAPCS.
     __ASM(
-        "LDR     r1, [r0]\n"
+        "LDR     r4, [r0]\n"
         "BX      lr\n"
     );
 }