mfd: Guard adp5520 PM methods with CONFIG_PM_SLEEP

This fixes below build warnings:
  CC      drivers/mfd/adp5520.o
drivers/mfd/adp5520.c:324:12: warning: 'adp5520_suspend' defined but not used [-Wunused-function]
drivers/mfd/adp5520.c:333:12: warning: 'adp5520_resume' defined but not used [-Wunused-function]

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>