PM / Domains: Introduce "always on" device flag

PM / Domains: Introduce "always on" device flag

This is a backport of mainline commit
1e78a0c7fc92aee076965d516cf54475c39e9894.

The TMU device on the Mackerel board belongs to the A4R power domain
and loses power when the domain is turned off.  Unfortunately, the
TMU driver is not prepared to cope with such situations and crashes
the system when that happens.  To work around this problem introduce
a new helper function, pm_genpd_dev_always_on(), allowing a device
driver to mark its device as "always on" in case it belongs to a PM
domain, which will make the generic PM domains core code avoid
powering off the domain containing the device, both at run time and
during system suspend.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Tested-by: Simon Horman <horms@verge.net.au>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>