serial: 8250_early: Add earlycon support for Tegra

Declare an OF early console for Tegra so that the early console device
can be specified via the "stdout-path" property in device-tree.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>