power_supply: Add support for writeable properties

This patch adds support for writeable power supply properties and
exposes them as writeable to sysfs.

A power supply implementation must implement two new function calls in
order to use that feature:

  int set_property(struct power_supply *psy,
                   enum power_supply_property psp,
                   const union power_supply_propval *val);

  int property_is_writeable(struct power_supply *psy,
                            enum power_supply_property psp);

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Alexey Starikovskiy <astarikovskiy@suse.de>
Cc: Len Brown <len.brown@intel.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Matt Reimer <mreimer@vpop.net>
Cc: Evgeniy Polyakov <zbr@ioremap.net>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>