spi: atmel-quadspi: add support for named peripheral clock

Naming clocks is a good practice. Keep supporting unnamed
peripheral clock, to be backward compatible with old DTs.
While here, rename clk to pclk, to indicate that it is a
peripheral clock.

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>