irqchip/gic: Drop support for board files

With the last non-OF, non-ACPI user of the GIC being removed in
e73307b9

 ("ARM: cns3xxx: remove entire platform"), we can finally
drop the entry point and do some minor cleanup.

We also make the driver depend on CONFIG_OF, which is required
even when CONFIG_ACPI is selected.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230315130218.3212033-1-maz@kernel.org