docs: marvell: update mv_ddr branch
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 <99c15d92ebc0e182e5c4df7c7e9e30ce96a863df@semihalf.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware