PM / OPP: Expose OPP tables to userspace via a device attribute

OPP tables may be particularly useful to userspace programs
that require voltage information in addition to frequency.
Expose the OPP table to userspace via a syfs node by adding
a device attribute when the first OPP entry is added to a
device.

Here's an example output:

\# cat /sys/devices/system/cpu/cpu0/opp_table
307200000 595000
422400000 620000
480000000 635000
556800000 650000
652800000 675000

The first column is the frequency and the second is
the corresponding voltage.

Change-Id: I0e2f3dbff8c7c55f5d821d8c40e054f695223fde
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>