arm64: zynqmp: Handle MMC seq number based on boot device

K26 has EMMC and SD and default 0 is not working when system is booting out
of SD which is controller 1. Add controller autodetection via
mmc_get_env_dev(). The same code is used for distro_boot selection done in
board_late_init(). bootseq variable can't be reused because this is called
so late.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>