clk: at91: sam9x60: fix usb clock parents

SAM9X60's USB clock has 3 parents: plla, upll and main_osc.

Fixes: 01e2113d ("clk: at91: add sam9x60 pmc driver")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lkml.kernel.org/r/1579261009-4573-3-git-send-email-claudiu.beznea@microchip.com


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