ASoC: cleanups and improvements for jz4740-i2s

Merge series from Aidan MacDonald <aidanmacdonald.0x0@gmail.com>:

This series is a preparatory cleanup of the jz4740-i2s driver before
adding support for a new SoC. The two improvements are lifting
unnecessary restrictions on sample rates and formats -- the existing
ones appear to be derived from the limitations of the JZ4740's internal
codec and don't reflect the actual capabilities of the I2S controller.

I'm unable to test the series on any JZ47xx SoCs, but I have tested
on an X1000 (which is the SoC I'll be adding in a followup series).