ARM: zynq: Define default SYS_PROMPT

All boards are using the same prompt that's why add it as default value to
Kconfig to simplify defconfigs.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 4e61565..78865eb 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -48,6 +48,7 @@
 
 config SYS_PROMPT
 	string "Shell prompt"
+	default "Zynq> " if ARCH_ZYNQ
 	default "=> "
 	help
 	  This string is displayed in the command line to the left of the