ARM: GIC: provide a single initialization function for boot CPU

Provide gic_init() which initializes the GIC distributor and current
CPU's GIC interface for the boot (or single) CPU.

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>