dt-bindings: spi: imx: Add an entry for the i.MX8QM compatible

Add an entry for the "fsl,imx8mq-ecspi" compatible to describe
the ECSPI version present on i.MX8M.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>