mmc: Support of PCI mode in the dw_mmc driver

Support of PCI mode for the dw_mmc driver. This Patch adds the
support for the scenario where the Synopsys Designware IP
is present on the PCI bus. The patch adds the minimal modifications
necessary for the driver to work on PCI platform. Also added separate
files for PCI and PLATFORM modes of operation.

Signed-off-by: Shashidhar Hiremath <shashidharh@vayavyalabs.com>
Acked-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>