drivers: sensors: add sensors ssc driver

SLPI (sensor low power island) uses Qualcomm Technologies, Inc
Digital Signal Processor for all sensors connected via SPI/I2C.
Firmware is required to be loaded to enable these sensors and
algorithms to execute on DSP. This change adds driver to load
the firmware.

This snapshot is taken as of msm-4.14 commit <2af01acb8e045>
("clk: qcom: gdsc: Remove global mutex lock in enable/disable
callbacks")' and minor modifications in Documentation.

Change-Id: I133b3abd31a860cff486f7e3aa7611a80b30fd58
Signed-off-by: Shaikh Shadul <sshadu@codeaurora.org>