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/rcar3_salvator-x_defconfig b/configs/rcar3_salvator-x_defconfig
index d0de58c..3b48ab2 100644
--- a/configs/rcar3_salvator-x_defconfig
+++ b/configs/rcar3_salvator-x_defconfig
@@ -3,6 +3,7 @@
 CONFIG_ARCH_RMOBILE=y
 CONFIG_ENV_SIZE=0x20000
 CONFIG_ENV_OFFSET=0xFFFE0000
+CONFIG_SYS_MALLOC_LEN=0x4000000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="r8a77950-salvator-x-u-boot"
 CONFIG_SPL_TEXT_BASE=0xe6338000