Convert CONFIG_SYS_MALLOC_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MALLOC_LEN

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/configs/stmark2_defconfig b/configs/stmark2_defconfig
index 24639cd..bca22c5 100644
--- a/configs/stmark2_defconfig
+++ b/configs/stmark2_defconfig
@@ -3,6 +3,7 @@
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0x40000
 CONFIG_ENV_SECT_SIZE=0x10000
+CONFIG_SYS_MALLOC_LEN=0x40000
 CONFIG_DEFAULT_DEVICE_TREE="stmark2"
 CONFIG_TARGET_STMARK2=y
 CONFIG_SYS_LOAD_ADDR=0x40010000