rtas instantiation when commandline contains mem

[Benjamin Krill]

Hi,

the rtas instantiation (prom_init.c) doesn't work correctly if the
kernel parameter "mem=" is used. The current code doesn't evaluate
the kernel parameter which causes the issue that alloc_down
allocates somewhere in the "real" memory space. So it can
happen that the allocation space is above "mem=".

Commit 2babf5c2ec2f2d5de3e38d20f7df7fd815fd10c9 removes the
evaluation of "mem=".

cheers
 ben