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/gwventana_defconfig b/configs/gwventana_defconfig
index ee823d2..c0904b1 100644
--- a/configs/gwventana_defconfig
+++ b/configs/gwventana_defconfig
@@ -55,3 +55,4 @@
 CONFIG_G_DNL_VENDOR_NUM=0x0525
 CONFIG_G_DNL_PRODUCT_NUM=0xa4a5
 CONFIG_OF_LIBFDT=y
+CONFIG_FDT_FIXUP_PARTITIONS=y