Do not allocate (and de-allocate) SPI DMA channels during setup
(and cleanup) for each SPI device. Instead, allocate the DMA
channels once duing probe and use them for the life time of
the driver.

This makes sense since there are dedicated DMA channels meant
for SPI use.

This also helps remove the unnecessary DMA "sync_dev" variables
being used to store DMA channel information.

Also, the "use_dma" platform variable is now eliminated since
it is possible to check if the platform supports DMA or not
based upon whether DMA resources can be found or not.
Tested-By: NMichael Williamson <michael.williamson@criticallink.com>
Tested-By: NBrian Niebuhr <bniebuhr@efjohnson.com>
Signed-off-by: NSekhar Nori <nsekhar@ti.com>