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.

For the Engicam-boards this means a split as the core
boards contains the emmc while the commit baseboard handles
sdmmc and sdio.
Suggested-by: Arnd Bergmann <arnd@kernel.org>
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Link: https://lore.kernel.org/r/20210324122235.1059292-3-heiko@sntech.de