clk: at91: add USB Host clock driver

Add USB clock driver to configure the input clock and the divider
in the PMC_USB register to generate a 48MHz and a 12MHz signal to
the USB Host OHCI.

Signed-off-by: Wenyou Yang <wenyou.yang@microchip.com>
diff --git a/drivers/clk/at91/Kconfig b/drivers/clk/at91/Kconfig
index fd56f20..8d482a2 100644
--- a/drivers/clk/at91/Kconfig
+++ b/drivers/clk/at91/Kconfig
@@ -27,6 +27,14 @@
 	  fast crystal oscillator to meet the frequency accuracy
 	  required by USB.
 
+config AT91_USB_CLK
+	bool "Support USB OHCI Input Clock"
+	depends on CLK_AT91
+	help
+	  This option is used to enable the USB Input Clock, from
+	  the device tree, configure the USBS bit (PLLA or UTMI PLL)
+	  and USBDIV field of the PMC_USB register.
+
 config AT91_H32MX
 	bool "Support H32MX 32-bit Matrix Clock"
 	depends on CLK_AT91