mfd: Fix sm501_register_gpio section mismatch

WARNING: drivers/mfd/built-in.o(.text+0x1706): Section mismatch in
reference from the function sm501_register_gpio() to the function
.devinit.text:sm501_gpio_register_chip()
The function sm501_register_gpio() references
the function __devinit sm501_gpio_register_chip().
This is often because sm501_register_gpio lacks a __devinit
annotation or the annotation of sm501_gpio_register_chip is wrong.

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>