gpio: davinci: move to platform device

Modify DaVinci GPIO driver to become a platform device
driver.

The driver does not have platform driver structure or
a probe. Instead, it has pure_initcall function for
initialization. The platform specific informaiton is
obtained using the DaVinci specific davinci_soc_info
structure. This is a problem for Device Tree (DT)
implementation.

As a first stage of DT conversion, we implement a probe.

Additional notes:

- The driver registration happens as  postcore_initcall.
  This is required since machine init functions like
  da850_lcd_hw_init() make use of GPIO.
- Start using devres APIs for simpler error handling.

Signed-off-by: KV Sujith <sujithkv@ti.com>
[avinashphilip@ti.com: Move global definition of
		       "davinci_gpio_controller" to local]
Signed-off-by: Philip Avinash <avinashphilip@ti.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
[nsekhar@ti.com: drop unused structure member, rebase to new
		 clean-up patch and fix error messages]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>