Documentation: devicetree: add pxa3xx compatible and syscon property

Document the new pxa3xx_nand driver compatible string for A7k/A8k SoCs
that need to access system controller registers in order to enable the
NAND controller through the use of a phandle pointed to by the
'marvell,system-controller' property.

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