driver: reset: spacemit-p1: add driver for poweroff/reboot

driver: reset: spacemit-p1: add driver for poweroff/reboot

This driver implements poweroff/reboot support for the SpacemiT P1 PMIC
chip, which is commonly paired with the SpacemiT K1 SoC.

The SpacemiT P1 support is implemented as a MFD driver, so the access is
done directly through the regmap interface. Reboot or poweroff is
triggered by setting a specific bit in a control register, which is
automatically cleared by the hardware afterwards.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Link: https://patch.msgid.link/20251102230352.914421-2-aurelien@aurel32.net
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>