isp1704_charger: Allow board specific powering routine

The ISP1704/1707 chip can be put to full power down
state by asserting the CHIP_SEL line. This patch enables
platform or board specific hooks to put the device into
power down mode in case not needed.

This patch is a preparation for enabling this powering
routine in n900 (rx-51) devices.

Thanks to Heikki Krogerus for helping out with the patch.

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@nokia.com>
Acked-By: Heikki Krogerus <heikki.krogerus@nokia.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>