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_imx7_defconfig b/configs/colibri_imx7_defconfig
index 5996cc9..25d7488 100644
--- a/configs/colibri_imx7_defconfig
+++ b/configs/colibri_imx7_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SYS_MEMTEST_END=0x8c000000
 CONFIG_ENV_SIZE=0x20000
 CONFIG_ENV_OFFSET=0x380000
+CONFIG_SYS_MALLOC_LEN=0x2000000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="imx7-colibri-rawnand"
 CONFIG_TARGET_COLIBRI_IMX7=y