configs: am3517_evm: Fix boot hang

SPL is really tight on space, so decrease a little memory that we
allocate in order to fix boot hang.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Tested-by: Derald D. Woods <woods.technical@gmail.com>
Link: https://lore.kernel.org/r/20210626134258.601331-1-aford173@gmail.com