ppc/e500: drop RAM size fixup

ppc/e500: drop RAM size fixup

If user provided non-sense RAM size, board will complain and
continue running with max RAM size supported.
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.

While at it, replace usage of global ram_size with
machine->ram_size

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: <20200219160953.13771-59-imammedo@redhat.com>