ASoC: kirkwood: allow smaller audio periods and smaller number of periods

There is no hardware restriction requiring a minimum of 8 periods, or
a minimum of 2048 bytes in a period.  Let's drop these values so that
userspace has more flexibility in choosing these parameters.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Mark Brown <broonie@linaro.org>