mpc85xx: fix WS-AP3710i boot

mpc85xx: fix WS-AP3710i boot

On master, the bootwrapper link-address for all simpleImage targets was
relocated to 0x15000000 due to growing kernel size.

This was not done on OpenWrt 23.05, as the decompressed kernel still
fits. However, with the wrapper for the WS-AP3710i, the bootloader
attempts execute in-place with the uImage load-address of 0x1000000. As
the image is compiled without the uImage header in mind, this naturally
fails.

In order to fix this, link the WS-AP3715i simpleImage at 0x15000000 as
done in master. This will force the bootloader to relocate the code to
the proper address and skip XIP.

Signed-off-by: David Bauer <mail@david-bauer.net>