board: stmark2: fix clock value

Fix totally blank console at boot, clock value must be decimal,
as for the 30Mhz external crystal.

Fixes: 26e5944ec90c ("stmark2: Migrate CONFIG_SYS_EXTRA_OPTIONS to Kconfig")
Signed-off-by: Angelo Dureghello <angelo@kernel-space.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/board/sysam/stmark2/Kconfig b/board/sysam/stmark2/Kconfig
index b259505..94f5049 100644
--- a/board/sysam/stmark2/Kconfig
+++ b/board/sysam/stmark2/Kconfig
@@ -11,7 +11,7 @@
 	depends on CF_SBF
 
 config SYS_INPUT_CLKSRC
-	hex
+	int "External crystal clock"
 	default 30000000
 
 config SYS_CPU