dt-bindings: spi: atmel-quadspi: make "pclk" mandatory

Naming clocks is a good practice. Make "pclk" madatory even if
we support unnamed clock in the driver, to be backward compatible
with old DTs.

Suggested-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>