powerpc/86xx: Enable XMC site on GE Fanuc SBC310

This patch enables the XMC (PCIe daughter card) site on the SBC310.
STG enter the description for the patch above.

Signed-off-by: Martyn Welch <martyn.welch@gefanuc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>