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/tqma6s_mba6_mmc_defconfig b/configs/tqma6s_mba6_mmc_defconfig
index 5c406fd..fce236e 100644
--- a/configs/tqma6s_mba6_mmc_defconfig
+++ b/configs/tqma6s_mba6_mmc_defconfig
@@ -5,6 +5,7 @@
 CONFIG_ENV_OFFSET=0x100000
 CONFIG_MX6S=y
 CONFIG_TARGET_TQMA6=y
+CONFIG_SYS_MALLOC_LEN=0x202000
 CONFIG_DEFAULT_DEVICE_TREE="imx6dl-mba6b"
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y