DA8xx: add generic GPIO driver

Add a generic GPIO driver for the DaVinci DA8xx processors. It is turned
on by defining CONFIG_DA8XX_GPIO and fulfills the generic GPIO interface
specified in <asm/gpio.h> . The driver has support for both manipulating
GPIO pins as well as automatically configuring the pin multiplexor
registers to set the pin function to GPIO.

Signed-off-by: Laurence Withers <lwithers@guralp.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>