ARM: dts: rockchip: Bump sd card pin drive strength up on firefly boards

It seems some firefly boards need 12mA drive strength for sdmmc.
Using 4mA/8mA drive strength will cause the kernel to fail to recognize
the sd card correctly.
Increased the sdmmc lines drive strength from 4mA to 12mA.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>