ARM: moxart: move DMA_OF selection to driver

Move the DMA_OF selection to the DMA driver to fix kconfig warning:

warning: (ARCH_MOXART) selects DMA_OF which has unmet direct dependencies (DMADEVICES && OF)

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Olof Johansson <olof@lixom.net>