devfreq: simple-dev: Make the freq-table property optional

The call to devfreq_add_device() tries to initialize frequency tables
for simple-dev. This can be a more efficient way of setting up
frequency tables in case there are multiple tables for a single device
depending on hardware version.
Make 'freq-tbl-khz' property an optional one so that probe doesn't
fail in its absence and devfreq_add_device() also gets a chance to
set up the freq table for simple-dev.

Change-Id: I96b5c2d4aef2085512d94dc6792f5ba2711de64b
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>