ENGR00292398 PXP: refine two spin locks usage in PXP dma driver

ENGR00292398 PXP: refine two spin locks usage in PXP dma driver

This patch provides the following refinements:
1. For pxp channel lock, use spin_lock() instead of spin_lock_irqsave().
   Since this lock is not used in any ISR. Moreover, this can increase the
   driver's concurrency with no local irq disabled.
2. Narrow down the pxp lock's locking range in pxp_issue_pending().
   Since this lock is also used in PXP ISR, so its hold time should be as
   few as possible to reduce the time when local irq disabled.

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