imx: mx6: add display of CPU temperature grade in print_cpuinfo()

When CONFIG_IMX6_THERMAL is defined print the CPU temperature grade info
along with the current temperature.

Before:
 CPU:   Temperature 42 C

After:
 CPU:   Automotive temperature grade (-40C to 125C) at 42C
 CPU:   Industrial temperature grade (-40C to 105C) at 42C
 CPU:   Extended Commercial temperature grade (-20C to 105C) at 42C

Cc: Stefan Roese <sr@denx.de>
Cc: Eric Nelson <eric.nelson@boundarydevices.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Jon Nettleton <jon.nettleton@gmail.com>
Cc: Jason Liu <r64343@freescale.com>
Cc: Ye Li <b37916@freescale.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: Markus Niebel <Markus.Niebel@tq-group.com>
Cc: Peng Fan <b51431@freescale.com>
Tested-by: Nikolay Dimitrov <picmaster@mail.bg>
Signed-off-by: Tim Harvey <tharvey@gateworks.com>