ARM: dts: k2g: Add keystone IRQ controller node

Add the Keystone IRQ controller IP node on Keystone 2 Galileo
SoC. This allows the ARM CorePac core to receive interrupts
from remote processor devices (eg: DSP) on the SoC.

The IP is identical in functionality to that of the equivalent
nodes on existing K2 SoCs. The only difference is the ARM INTC
interrupt id/event number.

Signed-off-by: Andrew F. Davis <afd@ti.com>
[s-anna@ti.com: revise commit description slightly]
Signed-off-by: Suman Anna <s-anna@ti.com>