arm: mach-k3: Add default ATF location for AM62/AM62a

There is a default ATF load address that is used for devices that have
ATF running in SRAM. For AM62 and AM62a, ATF runs from DRAM. Instead
of having to override the address in every defconfig, make add a
default for these ATF in DRAM devices.

Signed-off-by: Andrew Davis <afd@ti.com>
Acked-by: Bryan Brattlof <bb@ti.com>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>