configs: am65x_evm_a53: Increase malloc pool before relocation

The malloc pool used before relocation is getting tight leading to
out of memory errors doing certain DM-related calls. Since we are
running the A53 SPL out of DDR let's just go ahead and bump its size
as used in U-Boot proper as well as SPL (via Kconfig default value)
from 8KB to 32KB.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>