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>
diff --git a/drivers/clk/at91/Kconfig b/drivers/clk/at91/Kconfig
index 4abc802..4563892 100644
--- a/drivers/clk/at91/Kconfig
+++ b/drivers/clk/at91/Kconfig
@@ -61,3 +61,10 @@
 	help
 	  This option is used to enable the AT91 SAM9X60's PLL clock
 	  driver.
+
+config AT91_SAM9X60_USB
+	bool "USB Clock support for SAM9X60 SoCs"
+	depends on CLK_AT91
+	help
+	  This option is used to enable the AT91 SAM9X60's USB clock
+	  driver.