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/s5p4418_nanopi2_defconfig b/configs/s5p4418_nanopi2_defconfig
index 858f2ae..eb60989 100644
--- a/configs/s5p4418_nanopi2_defconfig
+++ b/configs/s5p4418_nanopi2_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SYS_MEMTEST_END=0xb0000000
 CONFIG_ENV_SIZE=0x4000
 CONFIG_ENV_OFFSET=0x2E0200
+CONFIG_SYS_MALLOC_LEN=0x2000000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="s5p4418-nanopi2"
 CONFIG_TARGET_NANOPI2=y