ASoC: bcm2835: add 24bit support

This adds 24 bit support to the I2S driver of the BCM2835

Code ported from bcm2708-i2s driver in Raspberry Pi tree.

Signed-off-by: Florian Meier <florian.meier@koalo.de>
Signed-off-by: Matthias Reichl <hias@horus.com>
Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>