cmd: kgdb: Enable kgdb only for PPC

Only PPC supports this option that's why there should be proper
dependency setup via Kconfig.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 7ed3c9c..d66f710 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1811,6 +1811,7 @@
 
 config CMD_KGDB
 	bool "kgdb - Allow debugging of U-Boot with gdb"
+	depends on PPC
 	help
 	  This enables a 'kgdb' command which allows gdb to connect to U-Boot
 	  over a serial link for debugging purposes. This allows