configs: at91: sam9x60: Add required configs for the USB clock

Add the configs required to use the SAM9X60's USB clock.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
[claudiu.beznea: added CONFIG_AT91_SAM9X60_USB to
 sam9x60_curiosity_mmc1_defconfig]
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>