It seems that the documentation does not reference this any more, plus,
bit 28 does not stick when written to the register.

Therefore I can only assume this is something which was documented in the
past but got removed from the hardware in the meantime.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181129134630.8222-1-tvrtko.ursulin@linux.intel.com