omap3: Reduce logic/overo SPL max image size

omap3: Reduce logic/overo SPL max image size

While the OMAP3 has 64KiB of SRAM, per the TRM the download area is only
from 0x40200000 to 0x4020F000 and exceeding that will cause failure to
boot.  Further, we need to make sure that we don't run into
SRAM_SCRATCH_SPACE_ADDR as once SPL is running we will write values
there and would corrupt our running image.

Cc: Adam Ford <aford173@gmail.com>
Cc: Steve Sakoman <sakoman@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>