soc: arm: npcx: move soc-specific register definitions to soc.h

This CL is to minimize `CONFIG_SOC_SERIES_XXXX` definitions when we
introduce a new chip series. Most of them are relevant to register
layouts in different npcx soc series. It moves soc-specific register
definitions from `reg_def.h` to its own soc.h file.

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