drivers: dp: fix build on M0 MCUs

Current code does not build on Cortex-M0, seems like it does not like
subs:

Error: instruction not supported in Thumb16 mode -- `subs r3,#1'

Adding a unified assembler language declaration in the snippet seems to
fix the problem, also add an M0+ board so this is tested in CI.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

Author: fabiobaltieri

drivers/dp/swdp_ll_pin.h

@@ -11,7 +11,8 @@
 static ALWAYS_INLINE void pin_delay_asm(uint32_t delay)
 {
 #if defined(CONFIG_CPU_CORTEX_M)
-	__asm volatile ("movs r3, %[p]\n"
+	__asm volatile (".syntax unified\n"
+			"movs r3, %[p]\n"
 			".start_%=:\n"
 			"subs r3, #1\n"
 			"bne .start_%=\n"

samples/subsys/dap/sample.yaml

@@ -9,4 +9,5 @@ tests:
     platform_allow:
       - nrf52840dk/nrf52840
       - frdm_k64f
+      - nucleo_c071rb
     tags: dap