arm: mvebu: Espressobin: Detect presence of emmc at runtime

Try to initialize emmc in board_late_init() and if it fails then we know
that emmc device is not connected.

This allows to use in U-Boot just one DTS file for all Espressobin variants
and also to correctly set fdtfile env variable for Linux kernel.

Signed-off-by: Pali Rohár <pali@kernel.org>
Tested-by: Gérald Kerma <gerald@gk2.net>
Reviewed-by: Andre Heider <a.heider@gmail.com>