mtd: rawnand: marvell: document a bit more the driver

A stale document about the old pxa3cc_nand.c driver is available in
Documentation/mtd/nand/. Rewrite the parts that explain the IP itself
and some non-trivial choices made in the driver directly in
marvell_nand.c to then be able to remove this file.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>