CLOCKSOURCE: mips-gic: Allow GIC clock to be specified in device-tree

As an alternative to the "clock-frequency" property, allow the GIC
timer operating clock to be specified in the device-tree instead.
This is useful on systems which use common clock or where the GIC
is not fixed to a particular frequency and is instead, for example,
derived from the CPU clock.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: devicetree@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/9309/