clk: imx8mp: fix root clock names for ecspi

Root clock name contained underscore, which does not match to the actual
clock name.

Correct the name to match what is present in the FDT.

Fixes: 87f95881 ("clk: imx8mp: Add ECSPI clocks")
Signed-off-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: uboot-imx <uboot-imx@nxp.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>