sunxi: dram: Add a helper function 'mctl_get_number_of_lanes'

It is going to be useful in more than one place.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>