powerpc/86xx: Fix GE SBC310 XMC site support

Correction to interrupt map mask for GE SBC310 XMC site and addition of
alias.

Signed-off-by: Malcolm Crossley <malcolm.crossley2@gefanuc.com>
Signed-off-by: Martyn Welch <martyn.welch@gefanuc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>