arm64: zynqmp: fix preprocessor check for SPL_ZYNQMP_TWO_SDHCI

A missing CONFIG_ prefix while checking for this Kconfig variable makes the
check always fail. Fix it. While there also switch from the '#if defined'
form to the '#ifdef' form as the other checks in this function.

Fixes: 35e2b923 ("arm64: zynqmp: Fix logic around CONFIG_ZYNQ_SDHCI")

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>