PCI: rcar: Move shareable code to a common file

Move shareable code to common file pcie-rcar.c and the #defines to
pcie-rcar.h so that the common code can be reused with endpoint driver.
There are no functional changes with this patch for the host controller
driver.

Link: https://lore.kernel.org/r/1588854799-13710-3-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com


Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>