ARM: dts: keystone: k2g: Rename the TI-SCI node

Lets rename the node name of TI-SCI node to be system-controller as it
is a better standardized name for the function that TI-SCI plays in
the SoC.

Reviewed-by: Tero Kristo <kristo@kernel.org>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>