omap: define _toggle_gpio_edge_triggering only for OMAP1

The only usage of _toggle_gpio_edge_triggering is in
an #ifdef CONFIG_ARCH_OMAP1 block, so only provide it if
CONFIG_ARCH_OMAP1 is defined, too.

This fixes a compiler warning:

	arch/arm/plat-omap/gpio.c:758: warning: '_toggle_gpio_edge_triggering' defined but not used

when compiling for ARCH_OMAP2, ARCH_OMAP3 or ARCH_OMAP4.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>