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/sandbox64_defconfig b/configs/sandbox64_defconfig
index 6b1d666..846417d 100644
--- a/configs/sandbox64_defconfig
+++ b/configs/sandbox64_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SYS_MEMTEST_START=0x00100000
 CONFIG_SYS_MEMTEST_END=0x00101000
 CONFIG_ENV_SIZE=0x2000
+CONFIG_SYS_MALLOC_LEN=0x2000000
 CONFIG_DEFAULT_DEVICE_TREE="sandbox64"
 CONFIG_PRE_CON_BUF_ADDR=0x100000
 CONFIG_BOOTSTAGE_STASH_ADDR=0x0