imx:mx6dlsabresd fix error detecting thermal

Before add CONFIG_SYS_MALLOC_F and CONFIG_SYS_MALLOC_F_LEN,
uboot will complains "CPU:   Temperature: Can't find sensor device".
This is because DM and DM_THERMAL are enabled, but SYS_MALLOC_F
is not configured.

After applying this patch, uboot can correctly detect the temperature.
"
U-Boot 2015.04-rc2-00146-g48b6e30-dirty (Mar 09 2015 - 13:04:36)

CPU:   Freescale i.MX6DL rev1.1 at 792 MHz
CPU:   Temperature 44 C
"

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>