powerpc/prom_init: Evaluate mem kernel parameter for early allocation

Evaluate mem kernel parameter for early memory allocations. If mem is set
no allocation in the region above the given boundary is allowed. The current
code doesn't take care about this and allocate memory above the given mem
boundary.

Signed-off-by: Benjamin Krill <ben@codiert.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>