mod/scmi_clock: Restructuring SCMI-clk module

The current implementation of agent table in SCMI-clock doesn't
follow the standard SCP framework. It is now restructured to do so,
i.e. each agent is treated as an element as the agent table is
initialized in the standard element_init() function.

Signed-off-by: Md Golam Tanvir Zim <mdgolamtanvir.zim@arm.com>
Change-Id: I424152ebaf9b43d2c6946aa38034c947ad088fb6