ARM: dts: keystone-k2g: Add ti-sci clock provider node

Add a ti-sci node representing the clock provider in the system.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>