gpio: Fix the no return statement warning

In commit 2425876 ("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 <[email protected]>
Signed-off-by: Kevin Hao <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Linus Walleij <[email protected]>

Author: haokexin

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 */