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/opos6uldev_defconfig b/configs/opos6uldev_defconfig
index 1e8d56e..714d991 100644
--- a/configs/opos6uldev_defconfig
+++ b/configs/opos6uldev_defconfig
@@ -9,6 +9,7 @@
 CONFIG_ENV_OFFSET=0x100000
 CONFIG_MX6UL=y
 CONFIG_TARGET_OPOS6ULDEV=y
+CONFIG_SYS_MALLOC_LEN=0x1000000
 CONFIG_DEFAULT_DEVICE_TREE="imx6ul-opos6uldev"
 CONFIG_SPL_TEXT_BASE=0x00908000
 CONFIG_SPL_MMC_SUPPORT=y