board: ti: am62x: am62x.env: Fix boot_targets

ti_mmc is not a valid boot_target for standard boot flow so
remove it. Prefer mmc1 (sd-card) over mmc0 (emmc).

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Nishanth Menon <nm@ti.com>