arm64: zynqmp: 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>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 78865eb..80849ba 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -49,6 +49,7 @@
 config SYS_PROMPT
 	string "Shell prompt"
 	default "Zynq> " if ARCH_ZYNQ
+	default "ZynqMP> " if ARCH_ZYNQMP
 	default "=> "
 	help
 	  This string is displayed in the command line to the left of the