gpio: Fix the no return statement warning

In commit 24258761 ("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.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/gpio/driver.h

@@ -549,12 +549,14 @@ static inline void *gpiochip_populate_parent_fwspec_twocell(struct gpio_chip *ch
 						    unsigned int parent_hwirq,
 						    unsigned int parent_type)
 {
+	return NULL;
 }
 
 static inline void *gpiochip_populate_parent_fwspec_fourcell(struct gpio_chip *chip,
 						     unsigned int parent_hwirq,
 						     unsigned int parent_type)
 {
+	return NULL;
 }
 
 #endif /* CONFIG_IRQ_DOMAIN_HIERARCHY */