docs: marvell: Update mv-ddr-marvell and A3700-utils-marvell branches

Marvell finally started providing the latest version of mv-ddr-marvell and
A3700-utils-marvell code in master branch of their git repositories.
Reflect this in build instructions.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I08d1189dac60eb2a28335c68f611c1da634106f6
1 file changed