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/meerkat96_defconfig b/configs/meerkat96_defconfig
index 7a92ba7..67934ae 100644
--- a/configs/meerkat96_defconfig
+++ b/configs/meerkat96_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SYS_MEMTEST_END=0xa0000000
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0x80000
+CONFIG_SYS_MALLOC_LEN=0x2000000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="imx7d-meerkat96"
 CONFIG_TARGET_MEERKAT96=y