ASoC: fsl_sai: mark regmap as fast_io

The regmap is only ever used to access MMIO registers, so it's fair
to say that register access is fast.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Link: https://lore.kernel.org/r/20190717105156.15721-1-l.stach@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>