arm64: dts: ti: k3-*: 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.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Tero Kristo <kristo@kernel.org>
Link: https://lore.kernel.org/r/20210510145033.7426-5-nm@ti.com