arm: zynq: Enable debug_uart_init in spl when enabled

In past this code was commented and was used for debug purpose.
But there is no reason not to enabled it based on macros.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/mach-zynq/spl.c

@@ -17,11 +17,12 @@ void board_init_f(ulong dummy)
 	ps7_init();
 
 	arch_cpu_init();
-	/*
-	 * The debug UART can be used from this point:
-	 * debug_uart_init();
-	 * printch('x');
-	 */
+
+#ifdef CONFIG_DEBUG_UART
+	/* Uart debug for sure */
+	debug_uart_init();
+	puts("Debug uart enabled\n"); /* or printch() */
+#endif
 }
 
 #ifdef CONFIG_SPL_BOARD_INIT