ENGR00293211 PXP: bind allocated DMA channels to opened device file descriptor

ENGR00293211 PXP: bind allocated DMA channels to opened device file descriptor

The allocated DMA channels via some opened file descriptor is better
to be bound to this descriptor. Since this can avoid some application
to fake a channel id which may be requested by other applications to
request PXP service. And also, this make it easier to release the dma
channel when application exists abnormally or forgets to release it
explicitly.

Signed-off-by: Fancy Fang <B47543@freescale.com>