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/stv0991_defconfig b/configs/stv0991_defconfig
index a05e991..37b10ba 100644
--- a/configs/stv0991_defconfig
+++ b/configs/stv0991_defconfig
@@ -1,3 +1,5 @@
 CONFIG_SYS_EXTRA_OPTIONS="stv0991"
 CONFIG_ARM=y
 CONFIG_TARGET_STV0991=y
+CONFIG_SYS_MALLOC_F=y
+CONFIG_SYS_MALLOC_F_LEN=0x2000