tegra2: Implement SPI / UART GPIO switch

tegra2: Implement SPI / UART GPIO switch

The Tegra2 Seaboard has the unfortunate feature that SPI and the console
UART are multiplexed on the same pins. We need to switch between one
and the other during SPI and console activity.

This new file implements a switch and keeps track of which peripheral
owns the pins. It also flips over the controlling GPIO as needed

Since we are adding a second file to board/nvidia/common, we create
a proper Makefile there and remove the direct board.o include from
board/nvidia/seaboard/Makefile

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