clk: at91: Add support for sam9x60 USB clock

Implement sam9x60 USB clock driver. This clock has
three parents: PLLA, UPLL and MAINXTAL. The driver is
aware of the three possible parents with the help of the
two mux tables provied to the driver during the registration
of the clock.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>