tegra2: spi: Support SPI / UART switch

Add the SPI / UART switch logic into the Tegra2 SPI driver so that it
can co-exist with the NS16550 UART.

We need the ns16550.h header for NS16550_t for now.

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