ARM: OMAP: Convert gpio-switch to platform_driver

Converts the gpio-switch driver to platform_driver to avoid bad cast
which occurs because the bus type is set to platform bus. Also
silences the following warnings:

arch/arm/plat-omap/gpio-switch.c:292: warning: ignoring return value
of 'device_create_file', declared with attribute warn_unused_result
arch/arm/plat-omap/gpio-switch.c:293: warning: ignoring return value
of 'device_create_file', declared with attribute warn_unused_result
arch/arm/plat-omap/gpio-switch.c:294: warning: ignoring return value
of 'device_create_file', declared with attribute warn_unused_result

Signed-off-by: Andrzej Zaborowski <balrog@zabor.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>