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/mx6memcal_defconfig b/configs/mx6memcal_defconfig
index 8cf0d13..f5b4c52 100644
--- a/configs/mx6memcal_defconfig
+++ b/configs/mx6memcal_defconfig
@@ -10,6 +10,7 @@
 CONFIG_MX6QDL=y
 CONFIG_MX6_DDRCAL=y
 CONFIG_TARGET_MX6MEMCAL=y
+CONFIG_SYS_MALLOC_LEN=0x4000000
 CONFIG_SPL_TEXT_BASE=0x00908000
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL=y