Add support to configure GIC-600's multichip routing table registers.
Introduce a new gic600 multichip structure in order to support platforms
to pass their GIC-600 multichip information such as routing table owner,
SPI blocks ownership.

This driver is currently experimental and the driver api may change in
the future.

Change-Id: Id409d0bc07843e271ead3fc2f6e3cb38b317878d
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>