arm64: dts: imx8mp: update ecspi compatible and clk

i.MX8MP ECSPI is derived from i.MX6UL, so update compatible
Add assigned-clocks settings

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>