ASoC: Xilinx fixes

Merge series from Robert Hancock <robert.hancock@calian.com>:

There are drivers in mainline for the Xilinx Audio Formatter and Xilinx
I2S IP cores. However, because of a few issues, these were only really
usable with Xilinx's xlnx_pl_snd_card top-level driver, which is not in
mainline (and not suitable for mainline).

The fixes in this patchset, for the simple-card layer as well as the
Xilinx drivers, now allow these drivers to be properly used with
simple-card without any out-of-tree support code.