Nuvoton: Remove dead code nu_delay_cycle_x4(...) ...

Originally, nu_delay_cycle_x4(...) is borrowed from mbed test code for delay
cycle. Currently, it is not used on Nuvoton targets. If delay cycle is needed,
use wait_ns(...) instead which has strict implementation and has passed tests.

[Nuvoton] Add nu_delay_cycle_x4 ...

nu_delay_cycle_x4 is a replacement for wait_us when us ticker is not available.

[Nuvoton] Add nu_busy_wait_us ...

nu_busy_wait_us is a replacement for wait_us when intermediary us ticker layer is disabled.

[Nuvoton] Move nu_countdown_init/expired/free implementations to nu_timer.c from nu_timer.h

Add system_reset() function to Mbed OS

[NUC472/M487] Fix indefinite loop in SHA alter.