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/evb-ast2600_defconfig b/configs/evb-ast2600_defconfig
index 4b6e596..b79e4a8 100644
--- a/configs/evb-ast2600_defconfig
+++ b/configs/evb-ast2600_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SYS_MALLOC_F_LEN=0x800
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x10000
+CONFIG_SYS_MALLOC_LEN=0x2000000
 CONFIG_DEFAULT_DEVICE_TREE="ast2600-evb"
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_SIZE_LIMIT=0x10000