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/trats2_defconfig b/configs/trats2_defconfig
index e1886dc..f630962 100644
--- a/configs/trats2_defconfig
+++ b/configs/trats2_defconfig
@@ -7,6 +7,7 @@
 CONFIG_TARGET_TRATS2=y
 CONFIG_ENV_SIZE=0x1000
 CONFIG_ENV_OFFSET=0x7000
+CONFIG_SYS_MALLOC_LEN=0x5001000
 CONFIG_DEFAULT_DEVICE_TREE="exynos4412-trats2"
 CONFIG_DISTRO_DEFAULTS=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set