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/r8a779a0_falcon_defconfig b/configs/r8a779a0_falcon_defconfig
index bb552f8..0151ca5 100644
--- a/configs/r8a779a0_falcon_defconfig
+++ b/configs/r8a779a0_falcon_defconfig
@@ -5,6 +5,7 @@
 CONFIG_ENV_SIZE=0x40000
 CONFIG_ENV_OFFSET=0xC00000
 CONFIG_ENV_SECT_SIZE=0x40000
+CONFIG_SYS_MALLOC_LEN=0x4000000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="r8a779a0-falcon-u-boot"
 CONFIG_SPL_TEXT_BASE=0xe6338000