drivers: spi: Microchip XEC QMSPI-LDMA fix spi buffer usage

Zephyr SPI driver model for full-duplex operation assumes
data will be transmitted and received during each clock period.
The QMSPI driver for the XEC family also supported dual and
quad I/O use cases which are inherently half-duplex. To
support dual/quad the driver incorrectly processed spi buffers
as all transmit buffers first then all receive buffers. This
worked if only the SPI driver was used. It did not work with
the Zephyr flash SPI NOR driver which assumes SPI drivers
follow the SPI driver model. This commit implements a QMSPI
driver that follows the Zephyr SPI driver model resulting in
a slightly smaller driver. Dual/quad SPI transactions are
supported if the experimental SPI extended mode Zephyr
configuration flag is enabled. We also remove the QMSPI full duplex
driver added previously to support the flash SPI NOR driver.
Added board to spi loop-back test and spi_flash sample.

Signed-off-by: Scott Worley <scott.worley@microchip.com>