arm/dt: msm8974: Add EPM device node

Embedded power measurement driver uses a Cypress
PSoC to measure voltage and current. The PSoC
is communicated over SPI.

The node adds the number of supported channels,
the individual channel's gain and rsense values
and the channel type. The rsense value is used
for current measurement and the channel type
lists the channel as current or voltage to the
EPM driver.

Change-Id: I0a53a9ff317e30f3d74ab88b62d55272f954f1e2
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>