rockchip: rk3288: fix EMMC_DIV_MASK definition in header

It should be '<<' instead of '<' for _MASK definition, fix it.

Signed-off-by: Ziyuan Xu <xzy.xu@rock-chips.com>
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>