mfd: Only register wm831x RTC device if the 32.768kHz crystal is enabled

The RTC uses the 32.768kHz crystal so if it's not enabled (and it can only
be enabled via OTP or InstantConfig, not runtime software) the RTC can't
function.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>