Convert CONFIG_SPL_DMA_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/platinum_picon_defconfig b/configs/platinum_picon_defconfig
index a960bf2..a0c122e 100644
--- a/configs/platinum_picon_defconfig
+++ b/configs/platinum_picon_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,MX6DL"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y
+CONFIG_SPL_DMA_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="picon > "
 CONFIG_CMD_BOOTZ=y