IRQCHIP: mips-gic: Add function for retrieving FDC IRQ

IRQCHIP: mips-gic: Add function for retrieving FDC IRQ

Add a function to the MIPS GIC driver for retrieving the Fast Debug
Channel (FDC) interrupt number, similar to the existing ones for the
timer and perf counter interrupts. This will be used by platform
implementations of get_c0_fdc_int() if a GIC is present.

A workaround exists for interAptiv and proAptiv which claim to be able
to route the FDC interrupt but don't seem to be able to in practice (at
least on Malta).

[ralf@linux-mips.org: Fix conflict.]

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/9142/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>