mx6sxsabresd: Add thermal support

Add thermal support so that the temperature of the chip can be displayed on
boot:

U-Boot 2015.01-rc1-18268-g1366c05-dirty (Nov 25 2014 - 13:02:42)

CPU:   Freescale i.MX6SX rev1.0 at 792 MHz
CPU:   Temperature 50 C

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>