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/brppt2_defconfig b/configs/brppt2_defconfig
index 246bbab..bcd46c1 100644
--- a/configs/brppt2_defconfig
+++ b/configs/brppt2_defconfig
@@ -14,6 +14,7 @@
 CONFIG_ENV_SECT_SIZE=0x10000
 CONFIG_MX6QDL=y
 CONFIG_TARGET_BRPPT2=y
+CONFIG_SYS_MALLOC_LEN=0xa00000
 CONFIG_DEFAULT_DEVICE_TREE="imx6dl-brppt2"
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SYS_BOOTCOUNT_ADDR=0x020CC068