mfd: tps6594: Add board power-off support

mfd: tps6594: Add board power-off support

Add a system level power-off handler if the "system-power-controller"
flag is set for this device in the device tree.

A power-off request is triggered by writing the TRIGGER_I2C_0 bit (which
is actually just a convention and really depends on the freely
programmable FSM).

Co-developed-by: Job Sava <jsava@criticallink.com>
Signed-off-by: Job Sava <jsava@criticallink.com>
Signed-off-by: Michael Walle <mwalle@kernel.org>
Link: https://lore.kernel.org/r/20250826134631.1499936-4-mwalle@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>