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/myir_mys_6ulx_defconfig b/configs/myir_mys_6ulx_defconfig
index 639e566..c3cce32 100644
--- a/configs/myir_mys_6ulx_defconfig
+++ b/configs/myir_mys_6ulx_defconfig
@@ -9,6 +9,7 @@
 CONFIG_ENV_SIZE=0x4000
 CONFIG_MX6ULL=y
 CONFIG_TARGET_MYS_6ULX=y
+CONFIG_SYS_MALLOC_LEN=0x1000000
 CONFIG_DEFAULT_DEVICE_TREE="imx6ull-myir-mys-6ulx-eval"
 CONFIG_SPL_TEXT_BASE=0x908000
 CONFIG_SPL_MMC_SUPPORT=y