ath79: GL-AR750S: Add I2C Support

ath79: GL-AR750S: Add I2C Support

The GL-AR750S has an internal header for I2C.

Provide DTS definitions for the i2c-gpio driver.

The I2C drivers; kmod-i2c-core, kmod-i2c-gpio
consume ~20 kB of flash and can be loaded as modules,

Default clock measured ~11.2 ms period, ~89 kHz

The board has well-labeled (unpopulated) headers for serial and I2C
along the front edge of the board (the edge with the LEDs). Looking
from the top, rear of the unit (behind Ethernet jacks)

  -------------------------------------------
       5G_LED        2G_LED      PWR_LED

                 O O O O O O O

                 3 S S G T R G
                 V C D N X X N
                 3 L A D     D

Signed-off-by: Jeff Kletsky <git-commits@allycomm.com>