arm64: dts: rockchip: enable sdmmc controller on rk3399-firefly

This allows basic support for SD highspeed cards but no UHS-I mode
got ready due to the propagated defer-probe error from RK805.

Cc: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>