x86: quark: Add routines to access message bus registers

x86: quark: Add routines to access message bus registers

In the Quark SoC, some chipset commands are accomplished by utilizing
the internal message network within the host bridge (D0:F0). Accesses
to this network are accomplished by populating the message control
register (MCR), Message Control Register eXtension (MCRX) and the
message data register (MDR).

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