axs10x/emdk/hsdk/iot_dk: Implement board_mmc_getcd()

axs10x/emdk/hsdk/iot_dk: Implement board_mmc_getcd()

So now we may detect MMC/SD-card existence and
instead of completely misleading message on missing card:
------------------------>8-----------------------
Loading Environment from FAT... Card did not respond to voltage select!
------------------------>8-----------------------

we now get very clear one:
------------------------>8-----------------------
Loading Environment from FAT... MMC: no card present
------------------------>8-----------------------

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>