dm: Kconfig: Move CONFIG_SYS_MALLOC_F_LEN to Kconfig

Move this option to Kconfig and update all boards.
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/gwventana_defconfig b/configs/gwventana_defconfig
index 4cddbdd..5b1a4c4 100644
--- a/configs/gwventana_defconfig
+++ b/configs/gwventana_defconfig
@@ -2,3 +2,5 @@
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,MX6QDL"
 +S:CONFIG_ARM=y
 +S:CONFIG_TARGET_GW_VENTANA=y
+CONFIG_SYS_MALLOC_F=y
+CONFIG_SYS_MALLOC_F_LEN=0x400