apalis-tk1: disable external clock loopback on SDMMC3

Actually make use of that shiny new CONFIG_TEGRA124_MMC_DISABLE_EXT_LOOPBACK.
Signed-off-by: NMarcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: NMarcel Ziswiler <marcel@ziswiler.com>
Signed-off-by: NTom Warren <twarren@nvidia.com>

configs/apalis-tk1_defconfig

@@ -28,6 +28,7 @@ CONFIG_SPL_DM=y
 CONFIG_DFU_MMC=y
 CONFIG_DFU_RAM=y
 # CONFIG_DM_MMC_OPS is not set
+CONFIG_TEGRA124_MMC_DISABLE_EXT_LOOPBACK=y
 CONFIG_E1000=y
 CONFIG_PCI=y
 CONFIG_DM_PCI=y