arm64: dts: rockchip: move mmc aliases to board dts on rk3308

As suggested by Arnd Bergmann, the newly added mmc aliases
should be board specific, so move them from the general dtsi
to the individual boards.

Suggested-by: Arnd Bergmann <arnd@kernel.org>
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Link: https://lore.kernel.org/r/20210324122235.1059292-4-heiko@sntech.de