Convert CONFIG_SPL_MPC8XXX_INIT_DDR_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/ls1043aqds_nand_defconfig b/configs/ls1043aqds_nand_defconfig
index 00b0531..10c176a 100644
--- a/configs/ls1043aqds_nand_defconfig
+++ b/configs/ls1043aqds_nand_defconfig
@@ -13,6 +13,7 @@
 CONFIG_NAND_BOOT=y
 CONFIG_BOOTDELAY=10
 CONFIG_SPL=y
+CONFIG_SPL_MPC8XXX_INIT_DDR_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_GREPENV=y