tegra2: Remove unused low-level Tegra2 UART code

This was used by the AVP in early boot but is no longer used. Unless we
plan to enable it somehow it is not needed. In any case we should try
to use the ns16550 driver instead as it has the same code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>