ARM: dts: k2g: Add DSP GPIO controller node

Add the DSP GPIO controller node on Keystone 2 Galileo SoC.
This is used to send interrupts to the only DSP processor
subsystem present on the SoC. The IP is identical to that
of the equivalent nodes on existing K2 SoCs.

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>