arm64: dts: sparx5: Add Sparx5 eMMC support

This adds eMMC support to the applicable Sparx5 board configuration
files.

Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Link: https://lore.kernel.org/r/20200825081357.32354-4-lars.povlsen@microchip.com