docs: arm: marvell: clarify some unimportant Armada 6x0 details

MMP2 is used in XO-1.75 and MMP3 is now supported in mainline.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20210215220839.423709-4-lkundrak@v3.sk


Signed-off-by: Jonathan Corbet <corbet@lwn.net>