arm64: zynqmp: Setup the first boot_target at run time

arm64: zynqmp: Setup the first boot_target at run time

Detect mmc alias at run time for setting up proper boot_targets sequence.
The first target has to correspond with boot mode.

The purpose of this patch is to get rid of CONFIG_ZYNQ_SDHCI0/1
parameters in full U-Boot.
Unfortunately this patch can't remove it because there is missing
mmc implementation for SPL_DM_SEQ_ALIAS.

Also xilinx_zynqmp.h only setup boot commands for mmc0 and mmc1.
It means using aliases with higher number won't work. But switching
between mmc0 and mmc1 should work properly.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Alexander Graf <agraf@suse.de>