spi: mxc_spi: Add imx6ul-ecspi compatible string

Recent i.MX8MP DTs use new fsl,imx6ul-ecspi compatible string instead
of the fsl,imx51-ecspi compatible string. Add the new compatible string
to fix ECSPI operation on i.MX8MP.

For details, see Linux:
48d74376fb68 ("arm64: dts: imx8mp: update ecspi compatible and clk")
8eb1252bbedf ("spi: imx: remove ERR009165 workaround on i.mx6ul")

Fixes: 451799a6 ("arm: dts: imx8mp: Sync the DT with kernel 6.4-rc4")
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Marek Vasut <marex@denx.de>