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/colibri_vf_defconfig b/configs/colibri_vf_defconfig
index 0dfa850..3fb13ea 100644
--- a/configs/colibri_vf_defconfig
+++ b/configs/colibri_vf_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SYS_MEMTEST_END=0x87c00000
 CONFIG_ENV_SIZE=0x20000
 CONFIG_ENV_OFFSET=0x180000
+CONFIG_SYS_MALLOC_LEN=0x0220000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="vf610-colibri"
 CONFIG_TARGET_COLIBRI_VF=y