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/T1024RDB_SPIFLASH_defconfig b/configs/T1024RDB_SPIFLASH_defconfig
index cdadfdf..1c25d21 100644
--- a/configs/T1024RDB_SPIFLASH_defconfig
+++ b/configs/T1024RDB_SPIFLASH_defconfig
@@ -7,6 +7,7 @@
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0x100000
 CONFIG_ENV_SECT_SIZE=0x10000
+CONFIG_SYS_MALLOC_LEN=0xa00000
 CONFIG_DEFAULT_DEVICE_TREE="t1024rdb"
 CONFIG_SPL_TEXT_BASE=0xFFFD8000
 CONFIG_SPL_SERIAL_SUPPORT=y