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/P1022DS_SDCARD_defconfig b/configs/P1022DS_SDCARD_defconfig
index 3acbc0a..ced8040 100644
--- a/configs/P1022DS_SDCARD_defconfig
+++ b/configs/P1022DS_SDCARD_defconfig
@@ -13,6 +13,7 @@
 CONFIG_SYS_EXTRA_OPTIONS="SDCARD"
 CONFIG_BOOTDELAY=10
 CONFIG_SPL=y
+CONFIG_SPL_MPC8XXX_INIT_DDR_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_SF=y