docs(linux): U-Boot: Falcon-Mode: document mmcdev

In OSPI boot with falcon mode, the 'mmcdev' variable is used to select
between SD/MMC or eMMC for loading the fitImage which wasn't documented
previously.

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

Author: ArchUsr64

source/linux/Foundational_Components/U-Boot/UG-Falcon-Mode.rst

@@ -75,7 +75,8 @@ OSPI boot:
 
    For OSPI boot, the ``tiboot3.bin`` and ``tifalcon.bin`` files should be
    flashed to the same addresses in flash as regular boot flow but the DTB and
-   the ``fitImage`` are read from the rootfs's boot directory.
+   the ``fitImage`` are read from the rootfs's boot directory. The MMC device
+   is selected by the ``mmcdev`` env variable for R5 SPL.
 
    Below U-Boot commands can be used to download ``tiboot3.bin`` and
    ``tifalcon.bin`` over tftp and then flash those to OSPI at their respective