Blackfin arch: add supporting for kgdb

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>

diff --git a/arch/blackfin/Kconfig.debug b/arch/blackfin/Kconfig.debug
index 0afa898..2cb0a30 100644
--- a/arch/blackfin/Kconfig.debug
+++ b/arch/blackfin/Kconfig.debug
@@ -2,6 +2,9 @@
 
 source "lib/Kconfig.debug"
 
+config HAVE_ARCH_KGDB
+       def_bool y
+
 config DEBUG_MMRS
 	bool "Generate Blackfin MMR tree"
 	select DEBUG_FS