arm64: dts: uniphier: enable more serial ports for PXs3 ref board

The serial pins of PXs3 SoC are not multiplexed with any other
functions.  Enable serial2 and serial3 on the PXs3 reference board
because I see the connectors on the board.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>