[ARM] 4753/1: [AT91] Use DMA_BIT_MASK

Replace hard-coded DMA mask (0xffffffff) with DMA_BIT_MASK(32) as
defined in dma-mapping.h.
Set "dma_mask" field for the UART platform_devices.

Signed-off-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>