Add support for enabling MCU controlled mode of the Turris Omnia LEDs
via a LED private trigger called "omnia-mcu". Recall that private LED
triggers will only be listed in the sysfs trigger file for LEDs that
support them (currently there is no user of this mechanism).

When in MCU controlled mode, the user can still set LED color, but the
blinking is done by MCU, which does different things for different LEDs:
- WAN LED is blinked according to the LED[0] pin of the WAN PHY
- LAN LEDs are blinked according to the LED[0] output of the
  corresponding port of the LAN switch
- PCIe LEDs are blinked according to the logical OR of the MiniPCIe port
  LED pins

In the future I want to make the netdev trigger to transparently offload
the blinking to the HW if user sets compatible settings for the netdev
trigger (for LEDs associated with network devices).
There was some work on this already, and hopefully we will be able to
complete it sometime, but for now there are still multiple blockers for
this, and even if there weren't, we still would not be able to configure
HW controlled mode for the LEDs associated with MiniPCIe ports.

In the meantime let's support HW controlled mode via the private LED
trigger mechanism. If, in the future, we manage to complete the netdev
trigger offloading, we can still keep this private trigger for backwards
compatibility, if needed.

We also set "omnia-mcu" to cdev->default_trigger, so that the MCU keeps
control until the user first wants to take over it. If a different
default trigger is specified in device-tree via the
'linux,default-trigger' property, LED class will overwrite
cdev->default_trigger, and so the DT property will be respected.
Signed-off-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/r/20230918161104.20860-4-kabel@kernel.orgSigned-off-by: Lee Jones <lee@kernel.org>