feat(sptool): add `physical-load-address` field to `sp_layout.json`

`physical-load-address` is now expected in `sp_layout.json` by
https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/19589

Signed-off-by: Karl Meakin <karl.meakin@arm.com>
Change-Id: I8df6c9bfab004f65e6ac3bc0467ade6dc3bf5ee2
1 file changed