ARM64: zynqmp: Fix bootmode SD_MODE1

When only sdhci1 IP is enabled and SD_MODE1 bootmode is selected
U-Boot using sdboot1 variable which refers to mmc dev 1.
But this device doesn't exist because only one controller is available.

This patch fix logic around sdboot mode with using sdbootdev internal
variable.

Reported-by: Chris Kohn <ckohn@xilinx.com>
Acked-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>