This adds support for runtime power management and, if configured, activates
automatic standby after 2 seconds of inactivity.

Inactivity means no read of acceleration values and no events triggered or
activated.

If CONFIG_PM is not set, this doesn't change anything for existing users.
Signed-off-by: NMartin Kepplinger <martink@posteo.de>
Signed-off-by: NChristoph Muellner <christoph.muellner@theobroma-systems.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>