soc: apple: rtkit: Add function to poweroff

soc: apple: rtkit: Add function to poweroff

Add a function to put a co-processor into the lowest possible power
state from which recovery usually isn't possible without a full SoC
reset. This is required for the USB4/Thunderbolt co-processors which
can be restarted since the entire USB4 root complex can be completely
reset independently of the rest of the SoC.

Reviewed-by: Janne Grunau <j@jannau.net>
Link: https://patch.msgid.link/20260117-apple-rtkit-poweroff-v2-1-b882a180e44d@kernel.org
Signed-off-by: Sven Peter <sven@kernel.org>