dt-bindings: clock: lan966x: Extend for clock gate support

Allow to add an optional resource to be able to access the clock gate
registers.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20211103085102.1656081-3-horatiu.vultur@microchip.com