ppc/{ppc440_bamboo, sam460ex}: drop RAM size fixup

ppc/{ppc440_bamboo, sam460ex}: drop RAM size fixup

If user provided non-sense RAM size, board will complain and
continue running with max RAM size supported or sometimes
crash like this:
  %QEMU -M bamboo -m 1
    exec.c:1926: find_ram_offset: Assertion `size != 0' failed.
    Aborted (core dumped)
Also RAM is going to be allocated by generic code, so it won't be
possible for board to fix things up for user.

Make it error message and exit to force user fix CLI,
instead of accepting non-sense CLI values.
That also fixes crash issue, since wrongly calculated size
isn't used to allocate RAM

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20200219160953.13771-66-imammedo@redhat.com>