arm: mvebu: Espressobin: Add support for emmc into dts file

To simplify setup, configuration and compilation of u-boot, define emmc
node for all Espressobin boards. Espressobin boards without populated emmc
works correctly, just detection and initialization of emmc obviously fails.

Code for emmc is extracted from commit f1a43c84 ("arm64: dts: a3720:
add support for espressobin with populated emmc").

Signed-off-by: Pali Rohár <pali@kernel.org>
Tested-by: Gérald Kerma <gerald@gk2.net>