mod/scmi_clock: Use physical index in scmi index lookup function

Updated `find_agent_scmi_clock_idx()` to take a physical device index
(`phy_dev_idx`) instead of a `fwk_id_t` clock id, for ease of use in
SCMI notification implementation.

No functional changes intended.

Signed-off-by: Md Golam Tanvir Zim <mdgolamtanvir.zim@arm.com>
Change-Id: I2890507b40b0056f0c2689ee6bc6114e7e8f6cf1