mach-k3: common: support only MMC in R5 falcon mode

mach-k3: common: support only MMC in R5 falcon mode

To simplify the boot process and prevent the R5 SPL size from growing,
this patch restricts the boot media to load the next stage payload
(tifalcon.bin and kernel FIT) to MMC only.

We select between eMMC/SD by checking "mmcdev" in env to conform with
how U-Boot proper handles loading binaries from MMC1 or MMC2.

Note that tiboot3.bin (the initial bootloader) can be loaded from any
boot mode supported by the ROM since the restriction only applies to
tifalcon.bin and fitImage.

Signed-off-by: Anshul Dalal <anshuld@ti.com>