ARM: dts: turris-omnia: Add MCU system-controller node

Turris Omnia's MCU provides various features that can be configured over
I2C at address 0x2a. Add device-tree node.

This does not carry a Fixes tag - we do not want this to get backported
to stable kernels for the following reason: U-Boot since v2022.10
inserts a phy-reset-gpio property into the WAN ethernet node pointing to
the MCU node if it finds the MCU node with a cznic,turris-omnia-mcu
compatible. Thus if this change got backported to a stable kernel, the
WAN interface driver would defer probe indefinitely (since it would wait
for the turris-omnia-mcu driver which would not be present).

Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20240701113010.16447-8-kabel@kernel.org


Signed-off-by: Arnd Bergmann <arnd@arndb.de>