am335x: configs: Use ISW_ENTRY_ADDR to set SPL_TEXT_BASE

The SPL load address changes based on boot type in HS devices,
ISW_ENTRY_ADDR is used to set this address for AM43xx based SoCs
for similar reasons. Add this same logic for AM33xx devices.

Also make the default value for ISW_ENTRY_ADDR correct for GP
devices based on SoC, HS devices already pick the correct
value in their defconfig.

Signed-off-by: Andrew F. Davis <afd@ti.com>