docs: marvell: update mv_ddr branch

Now that the BLE image sources (mv_ddr) are updated, reflect
the proper branch in the Armada build howto.

Change-Id: I959d1343d0dfdd681c7e39bdcaed9b36aaddfca1
Signed-off-by: Marcin Wojtas <mw@semihalf.com>
1 file changed