dm: tegra: Enable driver model in SPL and adjust the GPIO driver

Use the full driver model GPIO and serial drivers in SPL now that these are
supported. Since device tree is not available they will use platform data.

Remove the special SPL GPIO function as it is no longer needed.

This is all in one commit to maintain bisectability.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/nvidia/seaboard/seaboard.c b/board/nvidia/seaboard/seaboard.c
index 11472eb..25480e4 100644
--- a/board/nvidia/seaboard/seaboard.c
+++ b/board/nvidia/seaboard/seaboard.c
@@ -20,10 +20,8 @@
 void gpio_early_init_uart(void)
 {
 	/* Enable UART via GPIO_PI3 (port 8, bit 3) so serial console works */
-#ifndef CONFIG_SPL_BUILD
 	gpio_request(GPIO_PI3, NULL);
-#endif
-	tegra_spl_gpio_direction_output(GPIO_PI3, 0);
+	gpio_direction_output(GPIO_PI3, 0);
 }
 #endif