board: nuvoton: Use an event to replace last_stage_init() and fix build error

Followed the new style use event to replace last_stage_init().
Fixed build error After replace last_stage_init().
And remove CONFIG_SYS_SKIP_UART_INIT from defconfig,
system will reuse the setting from bootblock
and skip the baud rate setting.

Signed-off-by: Jim Liu <JJLIU0@nuvoton.com>