irqchip/gic: Add helper function for chip initialisation

For GICs that require runtime power-management it is necessary to
populate the 'parent_device' member of the irqchip structure. In
preparation for supporting such GICs, move the code that initialises
the irqchip structure for a GIC into its own function called
gic_init_chip() where the parent device pointer is also set.

Instead of calling gic_init_chip() from within gic_init_bases(), move
the calls to outside of this function, so that in the future we can
avoid having to pass additional parameters to gic_init_bases() in order
set the parent device pointer or set the name to a specific string.

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