imx: imx8mp_rsb3720a1: fix incorrect ifdef check on SPL_MMC

Since commit 103c5f18 ("mmc: Rename MMC_SUPPORT to MMC"),
SPL_MMC_SUPPORT is named SPL_MMC, so let's fix the ifdef.

Fixes: fbc6b141 ("imx: imx8mp_rsb3720a1: convert to DM_SERIAL")
Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>