tegra20: rework UART GPIO handling

Rename board provided gpio_config_uart() to
gpio_early_init_uart() as it does the same thing as the equally
called function provided by the uart-switch code. This allows
to simply call this function in early board init whether or not
we are building with CONFIG_UART_SWITCH defined.

Also provide a weak symbol for this function, to avoid the
need to provide this function for boards that don't need any
fixup.

This patch supersedes the earlier posted
"tegra: convert gpio_config_uart to weak symbol".
Build tested with MAKEALL -s tegra20

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>