This adds the PM8921 XOADC node to the PM8921 PMIC node,
defines the channels and further also define an IIO HWMON
node for the channels that are used for housekeeping of
voltages and die temperature for the PMIC chip die.

Tested on the Nexus 7 tablet:
lsiio
Device 000: PM8921-XOADC
cd /sys/bus/iio/devices/iio:device0
cat in_voltage10_input
616461  (0.625V reference voltage)
cat in_voltage11_input
 (1.25V reference voltage)
cat temp1_input
35852 (die temperature)

Cc: John Stultz <john.stultz@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>