ppc: Move malloc zone

ppc: Move malloc zone

ofmem was fixed at 0x05400000, followed by the malloc zone. The latter was
bounded by the stack (now client stack), relative to the top of RAM.
An increase of RAM would therefore only enlarge the malloc zone.

Move the malloc zone below the client stack, with a fixed size of 2 MiB.
The size is derived from the memory map depicted in ofmem.c;
having a fixed size leaves room for memory claim'ed by clients
and by OpenBIOS.

v2:
* Through the preceding patch the malloc zone goes below the client stack
  rather than below the stack. Adjust and prettify the illustration.

Cc: Alexander Graf <agraf@suse.de>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
git-svn-id: svn://openbios.org/openbios/trunk/openbios-devel@905 f158a5a8-5612-0410-a976-696ce0be7e32