mfd: arizona: Mark AIFx_TX_BCLK_RATE as readable for cs47l24

This register is used in the AIF code but is missing from the register
tables.

Signed-off-by: Praveen Kumar Vegivada <praveen.vegivada@cirrus.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>