x86: mmc: Move common FSP functions into a common file

Since these board functions seem to be the same for all boards which use
FSP, move them into a common file. We can adjust this later if future FSPs
need more flexibility.

This creates a generic PCI MMC device.

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