mfd: Guard max77686_pmic_dt_match with CONFIG_OF

This fixes below build warning if CONFIG_OF is not set.

  CC      drivers/mfd/max77686.o
drivers/mfd/max77686.c:37:42: warning: 'max77686_pmic_dt_match' defined but not used [-Wunused-variable]

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>