libhardware: Add audio power hint

Change-Id: I5b03cc060356da23b6941c5937ef217c70dbf6dd
libhardware: Define queriable attribute for max. WriteBack width

We define a new queriable attribute for the maximum width supported
by WriteBack.

Change-Id: I82a1fe6b10c5650a4a5358eab5c52d46bd5f19e3