mfd/gpio: Move HTC GPIO driver to GPIO subsystem

The HTC GPIO driver is a pure GPIO driver and I just can not
see what it is doing inside MFD. Let's just move it to GPIO
and take this opportunity to move the platform data to
<linux/platform_data/gpio-htc-egpio.h>

Cc: arm@kernel.org
Cc: Russell King <linux@armlinux.org.uk>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>