imx: mx6: display max cpu frequency in print_cpuinfo()

Display the max CPU frequency as well as the current running CPU frequency
if the max CPU frequency is available and differs from the current CPU
frequency.

Before:
CPU:   Freescale i.MX6Q rev1.2 at 792 MHz

After - using an 800MHz IMX6DL (running at its max)
CPU:   Freescale i.MX6DL rev1.1 at 792 MHz

After - using a 1GHz IMX6Q (not running at its max):
CPU:   Freescale i.MX6Q rev1.2 996 MHz (running at 792 MHz)

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>