gpio/omap: omap_gpio_init_context stub must be inline

The bug fix 352a2d5b "gpio/omap: ensure gpio context is initialised"
has caused a new warning for omap1_defconfig:

drivers/gpio/gpio-omap.c:1465:13: warning: 'omap_gpio_init_context' defined but not used [-Wunused-function]
 static void omap_gpio_init_context(struct gpio_bank *p) {}
             ^

The solution is to mark the stub function as 'static inline' so
it gets left out of the build when unused.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>