soc: npcx: extend group field of struct npcx_alt to 8-bit

In npcx9, the number of pinmux registers (DEVALTx) is more than 16. We
need to extend the "group" member in the struct npcx_ALT to configure
the pinmux settings in npcx9.

Signed-off-by: Jun Lin <CHLin56@nuvoton.com>
Signed-off-by: Mulin Chao <mlchao@nuvoton.com>