PM / Domains: remove define_genpd_open_function() and define_genpd_debugfs_fops()

We already have the DEFINE_SHOW_ATTRIBUTE, There is no need to define
such a macro, so remove define_genpd_open_function and
define_genpd_debugfs_fops.

Convert them to DEFINE_SHOW_ATTRIBUTE.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>