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/sandbox_defconfig b/configs/sandbox_defconfig
index 660063e..0bf5ea3 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -5,3 +5,5 @@
 CONFIG_FIT_SIGNATURE=y
 CONFIG_DM=y
 CONFIG_DEFAULT_DEVICE_TREE="sandbox"
+CONFIG_SYS_MALLOC_F=y
+CONFIG_SYS_MALLOC_F_LEN=0x400