x86: Use struct mrc_region to describe a mrc region

Currently struct fmap_entry is used to describe a mrc region.
However this structure contains some other fields that are not
related to mrc cache and causes confusion. Besides, it does not
include a base address field to store SPI flash's base address.
Instead in the mrccache.c it tries to use CONFIG_ROM_SIZE to
calculate the SPI flash base address, which unfortunately is
not 100% correct as CONFIG_ROM_SIZE may not match the whole
SPI flash size.

Define a new struct mrc_region and use it instead.

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