spi: Add fsl-dspi driver ...

The fsl-dspi is found on Freescale Vybrid, Coldfire and Layerscape SoCs.
This adds the driver based on the Linux driver as of v5.5. Coldfire
support has been dropped as it's rather obsolete. Vybrid support has
been dropped because it needs the DMA engine support we do not have in
barebox. What's left is support for Layerscape where this driver has
been tested on.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>