ARM: dts: rockchip: move rv1108 mmcx aliases to board dts files

As suggested by Arnd Bergmann, mmc-aliases are supposed to be
board-specific, so move the newly added general aliases to
the board-level on rv1108-based boards.

Suggested-by: Arnd Bergmann <arnd@kernel.org>
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Link: https://lore.kernel.org/r/20210324122235.1059292-2-heiko@sntech.de