ARM: GIC: consolidate gic_cpu_base_addr to common GIC code

Every architecture using the GIC has a gic_cpu_base_addr pointer for
GIC 0 for their entry assembly code to use to decode the cause of the
current interrupt.  Move this into the common GIC code.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>