mtd: nand: pxa3xx: Make dma code dependent on dma capable platforms

This patch adds a macro ARCH_HAS_DMA to compile-out arch specific
dma code, namely pxa_request_dma() and pxa_free_dma(). These symbols
are available only in pxa, which makes impossible to build the driver in
other platforms than ARCH_PXA.

In order to handle non-dma capable platforms, we implement a fallbacks that
allocate buffers as if 'use_dma=false', putting the dma related code
under the ARCH_HAS_DMA conditional.

Please note that the correct way to handle this is to migrate the
dma code to use of the mmp_pdma dmaengine driver. However, currently
this is not possible because the two dmaengine drivers can't work together.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>