clk: imx: Add ECSPI to iMX8MN

Add ECSPI clock entries to iMX8MN clock driver. Only make those entries
available in case SPI support in U-Boot is enabled at all to conserve
space, esp. in SPL.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>