arm64: zynqmp: spl: fix dual SD controller support

When enabling both SDHCI controllers, spl_mmc.c would actually choose
device sdhci0 even if booted from sdhci1 (boot_device). This is because
spl_mmc_get_device_index(boot_device) expects BOOT_DEVICE_MMC2[_2] in
order to return index 1 instead of 0.

The #if defined(...) statement is copied from board/xilinx/zynqmp/zynqmp.c

So the key to properly enabling both controllers as boot sources is
defining both CONFIG_ZYNQ_SDHCI0 and CONFIG_ZYNQ_SDHCI1 in your board's
include/configs/*.h.

Signed-off-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>