arm: mach-k3: am625: Relax emmc boot condition

spl_mmc_emmc_boot_partition return a number different from 0
if the partition is a boot one. We can have the uboot img
for instance in a raw offset in emmc partition 0 so we would
like to continue to load the next stage. If the user want
to use EMMC as boot device allow him to use any part of the
emmc and not only boot partition

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>