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/seeed_npi_imx6ull_defconfig b/configs/seeed_npi_imx6ull_defconfig
index bc6b4e8..c560aef 100644
--- a/configs/seeed_npi_imx6ull_defconfig
+++ b/configs/seeed_npi_imx6ull_defconfig
@@ -10,6 +10,7 @@
 CONFIG_ENV_OFFSET=0x3c00000
 CONFIG_MX6ULL=y
 CONFIG_TARGET_NPI_IMX6ULL=y
+CONFIG_SYS_MALLOC_LEN=0x0200000
 CONFIG_DEFAULT_DEVICE_TREE="imx6ull-seeed-npi-imx6ull-dev-board"
 CONFIG_SPL_TEXT_BASE=0x908000
 CONFIG_SPL_MMC_SUPPORT=y