dt-bindings: update grf-binding for rv1108 SoCs

This patch adds the compatible of GRF and USBGRF for RV1108 SoCs.

Signed-off-by: Frank Wang <frank.wang@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>