gpio: introduce GPIO_DAVINCI kconfig option

The compatible to Davinci GPIO HW block is used by other TI SoCs, like
Keystone, where GPIO support is declared as optional.

Hence, introduce GPIO_DAVINCI Kconfig option which will allow to enable
Davinci GPIO driver for Keystone SoCs when needed. At same time, kept
Davinci GPIO driver enabled for Davinci SoCs by default.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>