rockchip: Enable PCIe/M.2 on rock960 board

Due to board limitation some SSD's would work
on rock960 PCIe M.2 only with 1.8V IO domain.

So, this patch enables grf io_sel explicitly to
make PCIe/M.2 to work.

Cc: Tom Cubie <tom@radxa.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>