[ALSA] ASoC: Add support for BCLK based on (Rate * Chn * Word Size)

This patch adds support for the DAI BCLK to be generated by multiplying
Rate * Channels * Word Size (RCW).
This now gives 3 options for BCLK clocking and synchronisation :-
 1. BCLK = Rate * x
 2. BCLK = MCLK / x
 3. BCLK = Rate * Chn * Word Size.  (New)
Changes:-
 o Add support for RCW generation of BCLK
 o Update Documentation to include RCW.
 o Update DAI documentation for label = value DAI modes.
 o Add RCW support to wm8731, wm8750 and pxa2xx-i2s drivers.

Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>