ARM: dts: dra7-evm: Fix i2c3 pinmux and frequency

The I2C3 pins are taken from pads E21 (GPIO6_14) and
F20 (GPIO6_15). Use the right pinmux register and mode.

Also set the I2C3 bus frequency to a safer 400KHz than
3.4Mhz.

CC: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>