board: ti: am62x: am62x: Add splash screen env variables

Set splash screen related env variables. Default splash source is
set to mmc where user is expected to keep bmp in compressed format
with name ti.gz on first partition of mmc.

Splash file will be uncompressed to DDR at address 0x82000000 and
splash position is set to middle of screen.

Signed-off-by: Nikhil M Jain <n-jain1@ti.com>