mfd: arizona: Add ASYNC_SAMPLE_RATE_2 registers

Some arizona devices have a second asynchronous sample rate, add the
registers necessary to support this.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>