ARM: tegra: Add serial port labels to Tegra124 DT

These labels will be used to provide deterministic numbering of consoles
in a later patch.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
[treding@nvidia.com: drop aliases, reword commit message]
Signed-off-by: Thierry Reding <treding@nvidia.com>