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/zmx25_defconfig b/configs/zmx25_defconfig
index f459c05..b774eaf 100644
--- a/configs/zmx25_defconfig
+++ b/configs/zmx25_defconfig
@@ -7,6 +7,7 @@
 CONFIG_TARGET_ZMX25=y
 CONFIG_IMX_CONFIG=""
 CONFIG_SYS_LOAD_ADDR=0x80000000
+CONFIG_SYS_MALLOC_LEN=0x3f8000
 CONFIG_BOOTDELAY=5
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="boot in %d s\n"