Convert CONFIG_EXTRA_CLOCK to Kconfig

This converts the following to Kconfig:
   CONFIG_EXTRA_CLOCK

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/board/sysam/stmark2/Kconfig b/board/sysam/stmark2/Kconfig
index 4abcdb3..49d0274 100644
--- a/board/sysam/stmark2/Kconfig
+++ b/board/sysam/stmark2/Kconfig
@@ -3,6 +3,9 @@
 config CF_SBF
 	def_bool y
 
+config EXTRA_CLOCK
+	def_bool y
+
 config SYS_INPUT_CLKSRC
 	hex
 	default 30000000