configs: am62x: add SPL_MAX_SIZE back

This was regressed by the following commit and is required to build with
additional configs enabled.

Fixes: 14439cd7 ("configs: k3: make consistent bootcmd across all k3 socs")

Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
Tested-by: Nikhil M Jain <n-jain1@ti.com>