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/mx7ulp_evk_defconfig b/configs/mx7ulp_evk_defconfig
index 0da5b50..04ace67 100644
--- a/configs/mx7ulp_evk_defconfig
+++ b/configs/mx7ulp_evk_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SYS_MEMTEST_END=0x9e000000
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0xC0000
+CONFIG_SYS_MALLOC_LEN=0x800000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="imx7ulp-evk"
 CONFIG_TARGET_MX7ULP_EVK=y