clk: at91: usb: introduce num_parents in driver's structure

SAM9X60 USB clock may have up to 3 parents. Save the number of parents in
driver's data structure and validate against it when setting parent.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lkml.kernel.org/r/1579261009-4573-5-git-send-email-claudiu.beznea@microchip.com


Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>