arm64: zynqmp: Fix logic around CONFIG_ZYNQ_SDHCI

Replace SDHCI controller listing by Kconfig symbol to let SPL know that
this board is using multiple SDHCIs controllers.
Kconfig help message should explain why this is needed.
Origin symbols were used in full u-boot but with moving to distro boot
this was fixed already.

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