sh: pci: Initial PCI-Express support for SH7786 Urquell board.

This adds initial support for the PCI-Express module in the SH7786,
particularly as it relates to the urquell platform. Presently it is
only supported in root complex mode, with endpoint mode still requiring
more debugging. 29/32-bit mode and lane configurations are selectable via
board mode pins, and are otherwise fixed.

Only 4x and 1x PCI channels are presently handled, the PCI bridge still
requires additional debugging and stabilization in hardware.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>