x86: quark: Add clrbits, setbits, clrsetbits macros for message port access

On Intel Quark, lots of registers on the message port need be
programmed. Add handy clrbits, setbits, clrsetbits macros for
message port access.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>