The __gic_init() function is only called from gic_of_init() now that the
non-DT path has been removed. In order to simplify the code & aid
readability, fold __gic_init() into gic_of_init().

This provides us with the ability to return an error code, which
__gic_init() was previously unable to do. As such the irq_domain_add_*()
error paths are modified to print & return an error rather than panic().

[ralf@linux-mips.org: Resoled reject.]
Signed-off-by: NPaul Burton <paul.burton@imgtec.com>
Acked-by: NMarc Zyngier <marc.zyngier@arm.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17050/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>