ARM: at91/dt: usb_a9260: define crystals frequencies

Define Calao USB-A9260, USB-A9G20 and USB-A9G20-LPW main and slow crystals
frequencies.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>