ARM: dts: BCM5301X: Specify uart2 in the DT

The BCM53016 in the Meraki MR32 utilizes the third "uart2"
to connect to a on-board Bluetooth-LE 4.0 BCM20732 chip.

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