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/ls1021aqds_sdcard_qspi_defconfig b/configs/ls1021aqds_sdcard_qspi_defconfig
index d1b44f0..9606310 100644
--- a/configs/ls1021aqds_sdcard_qspi_defconfig
+++ b/configs/ls1021aqds_sdcard_qspi_defconfig
@@ -13,6 +13,7 @@
 CONFIG_SD_BOOT=y
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y
+CONFIG_SPL_MPC8XXX_INIT_DDR_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set