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/highbank_defconfig b/configs/highbank_defconfig
index b0f1e9e..43a070e 100644
--- a/configs/highbank_defconfig
+++ b/configs/highbank_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SYS_TEXT_BASE=0x00008000
 CONFIG_NR_DRAM_BANKS=2
 CONFIG_ENV_SIZE=0x2000
+CONFIG_SYS_MALLOC_LEN=0x80000
 CONFIG_SYS_BOOTCOUNT_ADDR=0xfff3cf0c
 CONFIG_SYS_BOOTCOUNT_SINGLEWORD=y
 CONFIG_DISTRO_DEFAULTS=y