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/ls1021atwr_sdcard_ifc_SECURE_BOOT_defconfig b/configs/ls1021atwr_sdcard_ifc_SECURE_BOOT_defconfig
index 95b21e7..a9d9a71 100644
--- a/configs/ls1021atwr_sdcard_ifc_SECURE_BOOT_defconfig
+++ b/configs/ls1021atwr_sdcard_ifc_SECURE_BOOT_defconfig
@@ -15,6 +15,7 @@
 CONFIG_SPL=y
 CONFIG_SPL_CRYPTO_SUPPORT=y
 CONFIG_SPL_HASH_SUPPORT=y
+CONFIG_SPL_MPC8XXX_INIT_DDR_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_GREPENV=y