gpio: Fix the no return statement warning

gpio: Fix the no return statement warning

In commit 242587616710 ("gpiolib: Add support for the irqdomain which
doesn't use irq_fwspec as arg") we have changed the return type of
gpiochip_populate_parent_fwspec_twocell/fourcell() from void to void *,
but forgot to add a return statement for these two dummy functions.
Add "return NULL" to fix the build warnings.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Kevin Hao <haokexin@gmail.com>
Link: https://lore.kernel.org/r/20200116095003.30324-1-haokexin@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>