irqchip: mips-gic: Add device-tree support

Add device-tree support for the MIPS GIC.  Update the GIC irqdomain's
xlate() callback to handle the three-cell specifier described in the
MIPS GIC binding document.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
---
Changes from v2:
 - updated for change in bindings
 - only parse first CPU vector
 - allow platforms to use EIC mode
Changes from v1:
 - updated for change in bindings
 - set base address and enable bit in GCR_GIC_BASE
clocksource: mips-gic: Add device-tree support

Parse the GIC timer frequency and interrupt from the device-tree.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>