irqchip/gic: Mark gic_init_physaddr() static

We get 1 warning when building kernel with W=1:
drivers/irqchip/irq-gic.c:917:13: warning: no previous prototype for 'gic_init_physaddr' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
so this patch marks this function with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>