spl: move FDT_FIXUP_PARTITIONS to Kconfig

Move FDT_FIXUP_PARTITIONS to Kconfig and cleanup existing
uses.

Signed-off-by: Heiko Schocher <hs@denx.de>
diff --git a/configs/cm_fx6_defconfig b/configs/cm_fx6_defconfig
index 998bc05..7c413d5 100644
--- a/configs/cm_fx6_defconfig
+++ b/configs/cm_fx6_defconfig
@@ -53,3 +53,4 @@
 CONFIG_USB_KEYBOARD=y
 CONFIG_SYS_USB_EVENT_POLL_VIA_INT_QUEUE=y
 CONFIG_OF_LIBFDT=y
+CONFIG_FDT_FIXUP_PARTITIONS=y