spi: atmel-quadspi: Add verbose debug facilities to monitor register accesses

This feature should not be enabled in release but can be useful for
developers who need to monitor register accesses at some specific places.

Helped me identify a bug in u-boot, by comparing the register accesses
from the u-boot driver with the ones from its linux variant.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
[jagan: use 16 bit array with tmp variable]
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>