ARM: dts: at91: sama5d2: Name the qspi clock

Naming clocks is a good practice. The atmel-quadspi driver supports
an unnamed clock for the peripheral clock in order to be backward
compatible with old DTs, but it is recommended to name the clocks
on new DTs. The driver's bindings file requires the clock-names
property, so name the clock.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20211209102542.254153-1-tudor.ambarus@microchip.com