LPC4088: Fix hardfault occuring after power-cycle

Since revision 5499db1 (mbed-os-5.6.0) a hardfault occurs after a power-cycle.
It doesn't occur after a reset when the application has been downloaded using
drag-and-drop or via debugger. This is probably the reason why this problem
isn't detected when testing new mbed releases.

The hardfault occured in hal_sleep(). Adding a __NOP after __WFI solves the
problem although I don't fully understand why.

- Revision ca661f9 is the last revision where the problem doesn't occur.
- The problem doesn't occur when compiling with GCC instead of ARM compiler
- This issue describes a similar, but not identical problem and led me to test adding a __NOP: https://github.com/ARMmbed/mbed-os/issues/5065

targets/TARGET_NXP/TARGET_LPC408X/sleep.c

@@ -25,6 +25,7 @@ void hal_sleep(void) {
 
     // wait for interrupt
     __WFI();
+    __NOP();
 }
 
 /*