ARM: davinci: da8xx: support gpio platform device

DaVinci GPIO driver now uses platform device model.
Convert DA8XX SoC code to use the new model.

Add da8xx_register_gpio() to create platform device for da8xx
platforms.

Signed-off-by: KV Sujith <sujithkv@ti.com>
Signed-off-by: Philip Avinash <avinashphilip@ti.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
[nsekhar@ti.com: simplify commit message]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>