firmware: imx: Add stub functions for SCMI CPU API

To ensure successful builds when CONFIG_IMX_SCMI_CPU_DRV is not enabled,
this patch adds static inline stub implementations for the following
functions:

  - scmi_imx_cpu_start()
  - scmi_imx_cpu_started()
  - scmi_imx_cpu_reset_vector_set()

These stubs return -EOPNOTSUPP to indicate that the functionality is not
supported in the current configuration. This avoids potential build or
link errors in code that conditionally calls these functions based on
feature availability.

Fixes: 1055faa5 ("firmware: imx: Add i.MX95 SCMI CPU driver")
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>