MAINTAINERS: rename 'Nuvoton platform' to 'Nuvoton_NPCX Platform'.

This CL renames 'Nuvoton Platform' as 'Nuvoton_NPCX Platform' and
modifies the relevant files used by this platform. It also changes
status from 'orphaned' to 'maintained' since we do maintain npcx drivers
continuously.

Signed-off-by: Mulin Chao <mlchao@nuvoton.com>