mfd: max77687: Add registration of max77686-clk

The MAX77686 clock driver has been in-tree for over 6 months, but never
actually enabled through the MFD registration before.

Add it to the table so the device will probe and configure properly.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>