power: Remove HAS_WAKELOCK config and document WAKELOCK

Remove the HAS_WAKELOCK config as it doesn't seem to have been used in
the 3.10 or 3.14 kernels.

Add some Documentation to CONFIG_WAKELOCK so that it is selectable and
can be disabled is desired.

Signed-off-by: Dylan Reid <dgreid@chromium.org>