env: ti: mmc.env: Move mmc related args to common place

All K3 SoCs use same set of args to load kernel for MMC. So move this to
common place to avoid duplication.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
Reviewed-by: Nikhil M Jain <n-jain1@ti.com>