ARM: VGIC: split gic.c to observe hardware/virtual GIC separation

ARM: VGIC: split gic.c to observe hardware/virtual GIC separation

Currently gic.c holds code to handle hardware IRQs as well as code to
bridge VGIC requests to the GIC virtualization hardware.
Despite being named gic.c, this file reaches into the VGIC and uses data
structures describing virtual IRQs.
To improve abstraction, move the VGIC functions into a separate file,
so that gic.c does what it says on the tin.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Acked-by: Julien Grall <julien.grall@arm.com>