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/sei510_defconfig b/configs/sei510_defconfig
index dde8d75..7e60922 100644
--- a/configs/sei510_defconfig
+++ b/configs/sei510_defconfig
@@ -6,6 +6,7 @@
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x10000
 CONFIG_ENV_OFFSET=0xFFFF0000
+CONFIG_SYS_MALLOC_LEN=0x8000000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="meson-g12a-sei510"
 CONFIG_MESON_G12A=y