For RK3399's GRF module, if we want to operate the graphic related grf
registers, we need to enable the pclk_vio_grf which supply power for VIO
GRF IOs, so it's better to introduce an optional grf clock in driver.
Signed-off-by: NYakir Yang <ykk@rock-chips.com>
Reviewed-by: NDouglas Anderson <dianders@chromium.org>
Reviewed-by: NTomasz Figa <tomasz.figa@chromium.com>