docs: marvell: Move Supported Marvell platforms to PLAT build option

Reformat list of boards, remove unsupported OcteonTX2 and mention
supported Turris MOX board.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I22cea7f77fd078554c7f0ed4108781626209e563
1 file changed