mach-k3: am62*: Fix backup from eMMC boot mode

mach-k3: am62*: Fix backup from eMMC boot mode

Currently logic in spl_mmc_boot_mode only lookes at main devstat
to determine the bootmode to return. Thus, when using: 'eMMC boot'
as primary boot mode and 'MMCSD boot from eMMC UDA' as backup
boot mode, 'eMMC boot' is always selected. Add check for bootindex
to determine if ROM boot via backup boot mode and return MMCSD_MODE_FS
which is the only supported backup bootmode with eMMC device.

Signed-off-by: Judith Mendez <jm@ti.com>
Reviewed-by: Anshul Dalal <anshuld@ti.com>
Reviewed-by: Moteen Shah <m-shah@ti.com>