ARM: imx: Let SPL configure ECSPI1 clock on Data Modul i.MX8M Plus eDM SBC

The SPL clock code does configure the ECSPI clock frequency, which has
to match the mxc-spi driver configuration for successful SPI NOR boot.
Drop the assigned-clock from DT ecspi1 node on this board to let the
SPL clock code do the configuration and keep it aligned with the driver
expectation.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>