ARM: dts: BCM5301X: Specify PWM in the DT

The BCM53016 in the Meraki MR32 uses the on-chip PWM
controller to drive a tri-color RGB LED. Since I plan
to use the PWM, I made a label for the pwm's pinmux
node. This way, it can be easily referenced.... And
Also included a label for the i2c since I'm going to
need it in the future too.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Acked-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>