mtd: hyperbus: hbmc-am654: Add DMA support for reads

AM654 HyperBus controller provides MMIO interface to read data from
flash. So add DMA memcpy support for reading data over MMIO interface.
This provides 5x improvement in throughput and reduces CPU usage as
well.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20200924081214.16934-5-vigneshr@ti.com