ARM: dts: lan966x: add clock gating register

The clock controller supports an optional clock gating register. This is
necessary to expose the USB device clock, for example. Add it.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220707132500.1708020-1-michael@walle.cc