configs: omap3x_logic: Fix boot hang by reducing SYS_MALLOC_F_LEN

The AM3517 uses SYS_MALLOC_F_LEN of size 0x3000, but the rest of
the OMAP3 boards from LogicPD / BeaconEmbedded use 0x4000, but
they don't boot SPL.

Reduce the malloc size to restore booting.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210625185717.243211-1-aford173@gmail.com