arm64: dts: rockchip: add sdhci/emmc for rk3399

Add description for the SDHCI v5.1 eMMC controller on rk3399. Fix it to
200 MHz, to support all supported timing modes.

Note that 'rockchip,rk3399-sdhci-5.1' is not documented; we presumably
have a compliant Arasan controller, but let's have a rockchip property
as the canonical backup/precautionary measure. Per Heiko's previous
suggestion, let's not clutter the arasan doc with it.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>