PM / devfreq: Use freq_table for available_frequencies

Some devices use freq_table instead of OPP. For those devices, the
available_frequencies file shows up empty. Fix that by using freq_table to
generate the available_frequencies data when OPP is not present.

Change-Id: Ibea8b388ee81c55d2eeddd8a1e2c18c91faed8c7
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>