ARM: mxc: rename gic_handle_irq to avoid name clash

Before introducing a global gic_handle_irq(), rename
MXC's version to mxc_gic_handle_irq(). This function will be
removed altogether in a later patch.

Cc: Sascha Hauer <kernel@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>