mod/scmi_clock: Refactor the clock states management

This patch refactors how the agent view of clock states are managed.
Previously, the clock states per agent were stored in a big array
under module context which would occupy unncessary memory not being
in use. In this change, the big array is broken down into smaller
arrays and put under agent context, occupying only necessary memory.

This patch also refactors and renames some clock state related
functions for improved efficiency and readability.

Change-Id: Iaf954165d9baaa570b91885b3367943468b95a6c
Signed-off-by: Md Golam Tanvir Zim <mdgolamtanvir.zim@arm.com>