hw/mcu/stm32: Fix WFI in RAM for -O3

Even though function stm32_wfi_from_ram() has attribute
to be placed in ram in -03 it can be inlined that can
effectively run WFI from flash.

Now additional attribute is added to make sure that
it is not inlined.

bx lr is also removed allowing compiler to make normal
exit from function (which in most cases will be bx lr)

Signed-off-by: Jerzy Kasenberg <jerzy@apache.org>

hw/mcu/stm/stm32_common/src/hal_os_tick.c

@@ -28,11 +28,10 @@
 #endif
 
 #if MYNEWT_VAL(STM32_WFI_FROM_RAM)
-__attribute__((section(".text_ram"))) void
+__attribute__((section(".text_ram"),noinline)) void
 stm32_wfi_from_ram(void)
 {
-    __ASM volatile ("wfi\n"
-                    "bx lr");
+    __ASM volatile ("wfi");
 }
 #endif