PM / devfreq: memlat: add device attribute to show core to device bw map

This adds the freq_map device attribute to the mem_latency governor in
order to display a given device's core frequency to device bandwidth.
The output should be printed in the formatted in the same way as the
example:

Core freq (MHz) Device BW
            300      1525
            480      3143
            900      4173
           1017      7759
           1296      9887
           1555     11863
           1804     13763

Change-Id: I6bef33a1239329f0687ee3983c2c02d84e984284
Signed-off-by: David Keitel <dkeitel@codeaurora.org>