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/M5235EVB_defconfig b/configs/M5235EVB_defconfig
index 4e0491c..d271144 100644
--- a/configs/M5235EVB_defconfig
+++ b/configs/M5235EVB_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SYS_TEXT_BASE=0xFFE00000
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_SECT_SIZE=0x2000
+CONFIG_SYS_MALLOC_LEN=0x20000
 CONFIG_DEFAULT_DEVICE_TREE="M5235EVB"
 CONFIG_TARGET_M5235EVB=y
 CONFIG_SYS_LOAD_ADDR=0x20000