Convert CONFIG_BOARD_POSTCLK_INIT to Kconfig

This converts the following to Kconfig:
   CONFIG_BOARD_POSTCLK_INIT

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/xtensa/Kconfig b/arch/xtensa/Kconfig
index 35e5b89..8f668cc 100644
--- a/arch/xtensa/Kconfig
+++ b/arch/xtensa/Kconfig
@@ -13,6 +13,7 @@
 
 config TARGET_XTFPGA
 	bool "Support XTFPGA"
+	select BOARD_POSTCLK_INIT
 
 endchoice