mfd: twl6040: Register child device for twl6040-pdmclk

The McPDM in OMAP4/5 is using the pdmclk from twl6040 as functional clock.
The twl6040-pdmclk driver provides a clock which can be used to make sure
that the pdmclk is active when the McPDM is in use.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>