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/P1020RDB-PC_NAND_defconfig b/configs/P1020RDB-PC_NAND_defconfig
index 76a3ddd..b4a59d3 100644
--- a/configs/P1020RDB-PC_NAND_defconfig
+++ b/configs/P1020RDB-PC_NAND_defconfig
@@ -13,6 +13,7 @@
 CONFIG_TPL_I2C_SUPPORT=y
 CONFIG_TPL_LIBCOMMON_SUPPORT=y
 CONFIG_TPL_LIBGENERIC_SUPPORT=y
+CONFIG_TPL_MPC8XXX_INIT_DDR_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_SF=y