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/nokia_rx51_defconfig b/configs/nokia_rx51_defconfig
index 20a51e1..1bb7664 100644
--- a/configs/nokia_rx51_defconfig
+++ b/configs/nokia_rx51_defconfig
@@ -4,3 +4,5 @@
 CONFIG_DM=n
 CONFIG_DM_SERIAL=n
 CONFIG_DM_GPIO=n
+CONFIG_SYS_MALLOC_F=y
+CONFIG_SYS_MALLOC_F_LEN=0x400