tegra2: Add support for Ventana

Ventana is a board which is very similar to Seaboard. Support it by
re-using board/nvidia/seaboard/seaboard.c with minor run-time conditionals.

v5: Makefile: Use cmd_link_o_target, remove unused clean/distclean targets.
v6: Make gpio_config_uart_seaboard() static.
v7: Add MAINTAINERS entry for Ventana. Tom Warren doesn't have Ventana, so
    he asked me to add myself for this board.

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