ASoC: kirkwood-i2s: add support for external clock rates

This is part of a patch found in Rabeeh Khoury's git tree for the
cubox, and cleaned up by me.

Some platforms provide an external clock which can be used to allow
other sample rates to be selected.  Provide support for this.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>