powerpc: Add GE FPGA config option

This patch adds the GE_FPGA configuration option. This is being carried
out as ground work to allow the PIC and GPIO drivers to be move from the
powerpc 86xx platform directory to more general locations to allow them to
be used on non-86xx boards and to reduce churn when further boards using
these drivers are added.

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