ppc/ppc405_boards: add RAM size checks

ppc/ppc405_boards: add RAM size checks

If user provided non-sense RAM size, board will ignore it
and continue running with fixed RAM size.

Also RAM is going to be allocated by generic code, so it
won't be possible for board to fix CLI.

Make it error message and exit to force user fix CLI,
instead of accepting non-sense CLI values.

PS:
move fixed RAM size into mc->default_ram_size, so that
generic code will know how much to allocate.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200219160953.13771-64-imammedo@redhat.com>