[PPC] Fix loader call to instantiate-rtas

OpenFirmware (OF) method instantiate-rtas was being called with a wrong
rtas-base-address argument. It must use the memory that is already being
allocated to this end instead. This issue was causing QEMU netboot to hang
when building the FDT from OF DT.

Reviewed by: jhibbits
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D24313