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/rpi_defconfig b/configs/rpi_defconfig
index 9379cf0..98d3199 100644
--- a/configs/rpi_defconfig
+++ b/configs/rpi_defconfig
@@ -1,2 +1,4 @@
 CONFIG_ARM=y
 CONFIG_TARGET_RPI=y
+CONFIG_SYS_MALLOC_F=y
+CONFIG_SYS_MALLOC_F_LEN=0x400