of: Add stub of_get_parent for non-OF builds

Fixes build error on x86_64 allmodconfig, introduced by commit
5ab3a89a ("mfd: syscon: Add non-DT support").

drivers/regulator/anatop-regulator.c: In function 'anatop_regulator_probe':
drivers/regulator/anatop-regulator.c:134:2: error: implicit declaration of function 'of_get_parent' [-Werror=implicit-function-declaration]

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>