barebox: force memory size to 64MB

This barebox version autodetects memory, but at installation time it
detects 8MB instead of 64MB.  This patch forces 64MB, like we used to
do in v2012.05 when no autodetection was there (actually, there the
pm9g45 harwired 128MB, but this had no side effects for us).

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>