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/pdm360ng_defconfig b/configs/pdm360ng_defconfig
index f3de685..9b8aa2f 100644
--- a/configs/pdm360ng_defconfig
+++ b/configs/pdm360ng_defconfig
@@ -13,3 +13,4 @@
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
 CONFIG_OF_LIBFDT=y
+CONFIG_FDT_FIXUP_PARTITIONS=y