ARM: dts: uniphier: renumber serial aliases for Gentil/Vodka boards

On these two boards, the serial0 is used for inter-chip connection,
so cannot be used for login console.  The serial2 is used instead
for them, but it is tedious to use because upper level deployment
projects must switch login console per board.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Olof Johansson <olof@lixom.net>