ARM: dts: keystone-k2g: Add PMMC node to support TI-SCI protocol

Texas Instrument's System Control Interface (TI-SCI) Message Protocol
is implemented in Keystone 2 generation 66AK2G SoC with the PMMC entity.

Add the ti-sci node representing this 66AK2G PMMC module.

Signed-off-by: Nishanth Menon <nm@ti.com>
[s-anna@ti.com: add unit address to DT node]
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>