soc: ti: knav_dma: rename pad in struct knav_dma_desc to sw_data

soc: ti: knav_dma: rename pad in struct knav_dma_desc to sw_data

Rename the pad to sw_data as per description of this field in the hardware
spec(refer sprugr9 from www.ti.com). Latest version of the document is
at http://www.ti.com/lit/ug/sprugr9h/sprugr9h.pdf and section 3.1
Host Packet Descriptor describes this field.

Define and use a constant for the size of sw_data field similar to
other fields in the struct for desc and document the sw_data field
in the header. As the sw_data is not touched by hw, it's type can be
changed to u32.

Rename the helpers to match with the updated dma desc field sw_data.

Cc: Wingman Kwok <[email protected]>
Cc: Mugunthan V N <[email protected]>
CC: Arnd Bergmann <[email protected]>
CC: Grygorii Strashko <[email protected]>
CC: David Laight <[email protected]>
Signed-off-by: Murali Karicheri <[email protected]>
Acked-by: Arnd Bergmann <[email protected]>
Signed-off-by: David S. Miller <[email protected]>