rockchip: elgin-rv1108: Use syscon API to get grf base

Use syscon API to get grf base instead of get from dts.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>