arm: mvebu: turris_omnia: Show MCU version

There are already more MCU firmware versions for Turris Omnia in
production, so display git commit (version) of the MCU firmware during
U-Boot startup. It will help to identify what version of MCU firmware is
Turris Omnia using.

MCU firmware for Turris Omnia is open source and available at website:
https://gitlab.nic.cz/turris/hw/omnia_hw_ctrl

It can be updated from running system via i2c bus with this tool:
https://gitlab.nic.cz/turris/omnia-mcutool



Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>