arm/tegra: Refactor board-*-pinmux.c to share code

This moves the implementation of *_pinmux_init() into a single location.
The board-specific pinmux data is left in each board's own file. This
will allow future changes that set up the pinmux in a more complex
fashion to do so without duplicating that code in each board's pinmux
file.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>