tegra: nyan-big: Enable the debug UART

Enable this to allow debugging when the serial UART driver is
misconfigured.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-on: Beaver, Jetson-TK1

configs/nyan-big_defconfig

@@ -46,6 +46,10 @@ CONFIG_PMIC_AS3722=y
 CONFIG_DM_REGULATOR=y
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_PWM_TEGRA=y
+CONFIG_DEBUG_UART=y
+CONFIG_DEBUG_UART_BASE=0x70006000
+CONFIG_DEBUG_UART_CLOCK=408000000
+CONFIG_DEBUG_UART_SHIFT=2
 CONFIG_SYS_NS16550=y
 CONFIG_TEGRA114_SPI=y
 CONFIG_TPM_TIS_INFINEON=y