dt-bindings: devfreq: Make the freq-table property optional

This patch the necessary documentation for the 'freq-tbl-khz'
property as 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: I715a7cc0b2d2b92ede44b1f5a0647f2e62382556