dt: lm3532: Add property for full scale current.

Add a property for each control bank to configure the
full scale current setting for the device.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>