sensors: ssc: update device tree documentation for ssc based sensors

Update documentation for ssc based sensors device tree to make
firmware name optional. Firmware is optional if sensors driver is
sharing processor.

Change-Id: Ia417f6fa47d7570a6560b1409fe61c0d22ad6aac
Signed-off-by: Ananda Kishore <kananda@codeaurora.org>