PM / wakeup: Use seq_open() to show wakeup stats

single_open() interface requires that the whole output must
fit into a single buffer. This will lead to timeout when
system memory is not in a good situation.

This patch use seq_open() to show wakeup stats. This method
need only one page, so timeout will not be observed.

Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>