spi: atmel-quadspi: Add support for classic SPI mode

The qspi controller on sama5d2 and sam9x60 supports "classic" SPI mode
without spi-mem enhancements and accelerations, very similar to the old
SPI controller on sam9g20 or the modern flexcom controllers of the same
SoC family.

Register interface differs somewhat, especially because only one
hardware controlled CS line is supported.  Some fields are missing, some
are in different registers, but in principal it works similar.  So code
is very much inspired by the old atmel-spi driver.

Tested on sam9x60 with a non-mainline driver to configure an FPGA.

Signed-off-by: Alexander Dahl <ada@thorsis.com>