dma: mmp_pdma: add protect when alloc/free phy channels

In mmp pdma, phy channels are allocated/freed dynamically
and frequently. But no proper protection is added.
Conflict will happen when multi-users are requesting phy
channels at the same time. Use spinlock to protect.

Signed-off-by: Xiang Wang <wangx@marvell.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>