arm: zynq: Support the debug UART

Add support for the debug UART to assist with early debugging. Enable it
for Zybo as an example.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/configs/zynq_zybo_defconfig b/configs/zynq_zybo_defconfig
index fc251dc..b7531d6 100644
--- a/configs/zynq_zybo_defconfig
+++ b/configs/zynq_zybo_defconfig
@@ -11,3 +11,7 @@
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_OF_SEPARATE=y
 CONFIG_NET_RANDOM_ETHADDR=y
+CONFIG_DEBUG_UART=y
+CONFIG_DEBUG_UART_ZYNQ=y
+CONFIG_DEBUG_UART_BASE=0xe0001000
+CONFIG_DEBUG_UART_CLOCK=50000000