spi: atmel-quadspi: add runtime pm support

Merge series from Claudiu Beznea <claudiu.beznea@microchip.com>:

The following series adds runtime PM support for atmel-quadspi driver.
clk_disable()/clk_enable() is called on proper
runtime_suspend()/runtime_resume() ops. Along with it 2 minor cleanups
were added (patches 2/3, 3/3).