omap: rx51: Add support for USB chargers

This enables isp1704 power supply driver on RX51, allowing
USB charger detection with N900.

Signed-off-by: Heikki Krogerus <ext-heikki.krogerus@nokia.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>