MIPS: BMIPS: dts: Add uart device nodes to bcm7xxx platforms

Add two uart device nodes known as the uart1 and uart2 for the bcm7xxx
platforms.

Signed-off-by: Jaedon Shin <jaedon.shin@gmail.com>
Cc: Kevin Cernekee <cernekee@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/9991/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>