mfd: twl-core: Enable regcache for audio registers

Enable regmap's regcache for the audio registers:
i2c address 0x49, register range 0x01 - 0x49
Mark all other registers as volatile to avoid any side effect for the non
audio functions behind 0x49 i2c address.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>