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/edison_defconfig b/configs/edison_defconfig
index 56cb816..53d5ce6 100644
--- a/configs/edison_defconfig
+++ b/configs/edison_defconfig
@@ -3,6 +3,7 @@
 CONFIG_NR_DRAM_BANKS=3
 CONFIG_ENV_SIZE=0x10000
 CONFIG_ENV_OFFSET=0x300000
+CONFIG_SYS_MALLOC_LEN=0x8000000
 CONFIG_DEFAULT_DEVICE_TREE="edison"
 CONFIG_ENV_OFFSET_REDUND=0x600000
 CONFIG_VENDOR_INTEL=y