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/variscite_dart6ul_defconfig b/configs/variscite_dart6ul_defconfig
index ef69d87..ab383ff 100644
--- a/configs/variscite_dart6ul_defconfig
+++ b/configs/variscite_dart6ul_defconfig
@@ -8,6 +8,7 @@
 CONFIG_ENV_SIZE=0x2000
 CONFIG_MX6ULL=y
 CONFIG_TARGET_DART_6UL=y
+CONFIG_SYS_MALLOC_LEN=0x1000000
 CONFIG_DEFAULT_DEVICE_TREE="imx6ull-dart-6ul"
 CONFIG_SPL_TEXT_BASE=0x00908000
 CONFIG_SPL_MMC_SUPPORT=y