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/durian_defconfig b/configs/durian_defconfig
index 2388d87..bc418f6 100644
--- a/configs/durian_defconfig
+++ b/configs/durian_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SYS_TEXT_BASE=0x500000
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x1000
+CONFIG_SYS_MALLOC_LEN=0x101000
 CONFIG_DEFAULT_DEVICE_TREE="phytium-durian"
 # CONFIG_PSCI_RESET is not set
 CONFIG_AHCI=y