sh: pci: Support asynchronous initialization of SH-X3 PCIe channels.

SH-X3 controllers all have pretty dire delays needed for PHY wakeup, so
we attempt to mitigate the damage by bringing them up asynchronously,
simply using the synchronization points for persistent bridge to channel
numbering.

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