ARM: dts: rockchip: Set sdmmc frequency at boot time for rk3066a

Currently driver leaves sdmmc frequency at its default.
So lets set this to 50MHz.
This gives us performance boost in mmc transfers.

Signed-off-by: Paweł Jarosz <paweljarosz3691@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>