dmaengine: at_hdmac: take maxburst from slave configuration

The maxburst/chunk size was taken from the private slave DMA data structure.
Use the common API provided by DMA_SLAVE_CONFIG to setup src/dst maxburst
values.
The ctrla field is not needed anymore in the slave private structure nor the
header constants that were located in an architecture specific directory.
The at91sam9g45_devices.c file that was using this platform data is also
modified to remove this now useless data.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>