irqchip/gic: Prepare for adding platform driver

irqchip/gic: Prepare for adding platform driver

To support GICs that require runtime power management, it is necessary
to add a platform driver, so that the probing of the chip can be
deferred if resources, such as a power-domain, is not yet available.

To prepare for adding a platform driver:
 1. Drop the __init section from the gic_dist_config() so this can be
    re-used by the platform driver.
 2. Add prototypes for functions required by the platform driver to the
    GIC header file so they can be re-used.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>