tegra2: Use new GPIO APIs in gpio_config_uart()

tegra2: Use new GPIO APIs in gpio_config_uart()

... rather than open-coding the register accesses.

However, gpio_request() typically stores the "label" parameter in a global
data structure. This causes problems when called from gpio_config_uart(),
since the code is running before relocation. To solve this, pass a NULL
string to gpio_request(), and modify gpio_request() not to touch the string
if it's NULL.

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