ARM: dts: tegra: move serial aliases to per-board

There are general changes pending to make the /aliases/serial* entries
number the serial ports on the system. On Tegra, so far the ports have
been just numbered dynamically as they are configured so that makes them
change.

To avoid this, add specific aliases per board to keep the old numbers.
This allows us to change the numbering by default on future SoCs while
keeping the numbering on existing boards.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Thierry Reding <treding@nvidia.com>