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/stv0991_defconfig b/configs/stv0991_defconfig
index d8008a8..b6ec831e 100644
--- a/configs/stv0991_defconfig
+++ b/configs/stv0991_defconfig
@@ -9,6 +9,7 @@
 CONFIG_ENV_SIZE=0x10000
 CONFIG_ENV_OFFSET=0x30000
 CONFIG_ENV_SECT_SIZE=0x10000
+CONFIG_SYS_MALLOC_LEN=0x14000
 CONFIG_DEFAULT_DEVICE_TREE="stv0991"
 CONFIG_SYS_LOAD_ADDR=0x0
 CONFIG_SYS_EXTRA_OPTIONS="STV0991"