ASoC: da7213: Add a set_bclk_ratio method

Following [1], it becomes harder for the CPU DAI to know the correct
BCLK ratio. We can either bake the same knowledge into the sound card
driver, or implement and use set_bclk_ratio on the codec. This commit
does the latter.

[1] commit c89e652e ("ASoC: da7213: Add support for mono, set
frame width to 32 when possible")

Signed-off-by: Phil Elwell <phil@raspberrypi.com>