rtc: twl: kill static variables

The current code uses static variables which prevent
the use of multiple rtc twl instances.
We also make it clear that this driver supports only
TWL4030 and TWL6030 classes.
Signed-off-by: Nicolae Rosia <Nicolae_Rosia@mentor.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>