sdhci: move pci stuff to separate module

The SDHCI interface is not PCI specific, yet the Linux driver was
intimitely connected to the PCI bus. This patch properly separates
the PCI specific portion from the bus independent code.

This patch is based on work by Ben Dooks but he did not have time
to complete it.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>