rnndb: add DMA debug register description

Info from imx6_v4_0_0 driver
driver: sanitize ETNA_PIPE usage to etna_pipe_context

We were abusing the priv field in struct pipe_context,
change this to use Mesa "inheritance" like for the other objects.
driver: compute component count for varyings

Find out which bits are used for PS inputs, and set the
varying configuration accordingly.
driver: remove newlines from DBG() messages