DTS: CI20: Fix bugs in ci20's device tree.

According to the Schematic, the hardware of ci20 leads to uart3,
but not to uart2. Uart2 is miswritten in the original code.

Signed-off-by: Zhou Yanjie <zhouyanjie@cduestc.edu.cn>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips <linux-mips@vger.kernel.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>
Cc: devicetree@vger.kernel.org
Cc: robh+dt@kernel.org
Cc: ralf@linux-mips.org
Cc: jhogan@kernel.org
Cc: mark.rutland@arm.com
Cc: malat@debian.org
Cc: ezequiel@collabora.co.uk
Cc: ulf.hansson@linaro.org
Cc: syq <syq@debian.org>
Cc: jiaxun.yang <jiaxun.yang@flygoat.com>