mfd: madera: Add support for requesting the supply clocks

Add the ability to get the clock for each clock input pin of the chip
and enable MCLK2 since that is expected to be a permanently enabled
32kHz clock.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>