msm: hwmon: Add EPM ADC driver

External Power Monitor(EPM) uses the external ADS1158 ADC
to measure voltage and current. Each ADS1158 ADC supports
measuring 16 Analog input channels.

APQ8064 has support for 2ADS ADC and can measure upto 32
analog input channels. The power to the EPM uses a
SX1509 I2C expander to enable the ADS ADC. Depending on
the input channel the appropriate chip select is de-asserted
to select the ADS ADC to measure the input signal. The
ADS1158 chip is configured through the SPI interface.

Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
Change-Id: Ib9f36606b6b183f75754b7763dbe45455c779327