a2dp hw: adding min freq boost for a2dp through bdroid_localcfg

Adding a simple line into your device tree's bdroid_buildcfg
elevates minfreq of the cpu governor while a2dp is being used.
This helps on some of the devices (like endeavoru) to use
a proper lowest frequency for decoding/encoding of the stream
going on in a2dp usage. It will let the min freq revert when
a2dp is finished. If the #define is not added the code is not
compiled in. The whole thing is added to this part (a2dp)
only, because other bluetooth operations doesnt need the
elevation of the min freq generally.

Example: in device's bdroid_buildcfg.h
 define A2DP_ENFORCE_MIN_FREQ "204000"

Change-Id: I2cd94e626eb2a6d04a6e52ddac94fa5773eb09d8