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/pepper_defconfig b/configs/pepper_defconfig
index 22c7bb4..14266ef 100644
--- a/configs/pepper_defconfig
+++ b/configs/pepper_defconfig
@@ -1,3 +1,5 @@
 CONFIG_SPL=y
 +S:CONFIG_ARM=y
 +S:CONFIG_TARGET_PEPPER=y
+CONFIG_SYS_MALLOC_F=y
+CONFIG_SYS_MALLOC_F_LEN=0x400