docs: marvell: update ddr3 build instructions

Add information about 2GB variant of EspressoBin V5 and use Marvell git
branches which contain required fixes for EspressoBin.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I1db510f1576f4762259ad7b0c10024b8ab434a59
1 file changed