soc: npcx: Add __packed to flags struct in npcx_pinctrl

When building with clang and CONFIG_LTO, clang warns:

soc/nuvoton/npcx/common/pinctrl_soc.h:141:4: error: field flags within
'struct npcx_pinctrl' is less aligned than 'struct (unnamed struct at
soc/nuvoton/npcx/common/./pinctrl_soc.h:128:2)' and is usually due to
'struct npcx_pinctrl' being packed, which can lead to unaligned accesses
[-Werror,-Wunaligned-access]
         } flags;
           ^

Signed-off-by: Tom Hughes <tomhughes@chromium.org>