atheros: simplify gpiolib realization

atheros: simplify gpiolib realization

Each SoCs generation has own independent gpiolib realization, so we
have no reason to keep these realizations in semiuniversal form.
Following modifications are made:
 * Remove valid_mask field
 * Remove ar231x_gpio_chip structure
 * Rename AR2315_GPIO_CR to AR2315_GPIO_DIR
 * Fix count of AR5312 GPIOs
 * Simplify gpio_chip methods realization

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41696 3c298f89-4303-0410-b956-a3cf2f4a3e73