tegra: Add a function mux feature

funcmux permits selection of config options for particular peripherals,
such as the pins that are used for that peripheral, if there are several
options.

Add UART selection to start with.

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