GPIO: OMAP: fix section mismatch warnings

WARNING: arch/arm/plat-omap/built-in.o(.devinit.text+0x46c): Section mismatch in reference from the function omap_gpio_probe() to the function .init.text:omap_gpio_chip_init()
The function __devinit omap_gpio_probe() references
a function __init omap_gpio_chip_init().
If omap_gpio_chip_init is only used by omap_gpio_probe then
annotate omap_gpio_chip_init with a matching annotation.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Kevin Hilman <khilman@ti.com>