[ARM] pxa/dma: add debugfs entries

Add some debug information for PXA DMA :
 - descriptors queued
 - channels state
 - global state

--
Since V1: reverted to old register access (no more dma_readl() or dma_writel()).

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>