arm: mvebu: Fix compatible string for nand controller

Linux kernel uses compatible string "marvell,armada370-nand-controller" for
nand controllers on Armada 370/XP/38x. U-Boot currently uses mix of
"marvell,armada370-nand" and "marvell,mvebu-pxa3xx-nand".

So unify it and use just Linux kernel compatible string.

Signed-off-by: Pali Rohár <pali@kernel.org>
Acked-by: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Stefan Roese <sr@denx.de>