mfd: add #ifdef CONFIG_DEBUG_FS guard for ab8500_debug_resources

Fix below build warning if CONFIG_DEBUG_FS is disabled.

  CC      drivers/mfd/ab8500-core.o
drivers/mfd/ab8500-core.c:623: warning: 'ab8500_debug_resources' defined but not used

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