dt-bindings: soc: rockchip: grf: add rockchip,rk3588-pmugrf

Add rockchip,rk3588-pmugrf compatible string.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lore.kernel.org/r/20231018061714.3553817-24-s.hauer@pengutronix.de
Signed-off-by: Heiko Stuebner <heiko@sntech.de>