ARM: GIC: move gic_data[] initialization into gic_init()

This avoids writing unnecessarily to gic_data[] from other CPUs,
making this a mostly read-only variable.

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>