pda_power: Add function callbacks for suspend and resume

Add function prototypes for power management events so they can be
handled and used by platform implementations.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Dmitry Baryshkov <dbaryshkov@gmail.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>