mtd: nand: pxa3xx_nand: enable building on mvebu 64-bit platforms

The controller supported by the pxa3xx_nand driver is also available on
the mvebu 64-bit SoCs, such as the Armada 7K and Armada 8K SoCs. This
patch updates the Kconfig dependency to allow building the kernel for
this SoC family too.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>