Davinci: ea20: added PREBOOT to configuration

PREBOOT is used on the ea20 to load a splash image
at the start up.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Sandeep Paulraj <s-paulraj@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>