boards: PHYTEC: phyboard_polis: remove mimx8mm prefix

For easier recognition PHYTEC boards have been prefixed with the SoC name.
As the new hardware model includes the SoC and cpu, this prefixing is not
needed anymore. All PHYTEC eval boards have an individual and unique name
and can be found easily via this name.

Signed-off-by: Jonas Remmert <j.remmert@phytec.de>