clk-si514, clk-si544: Implement prepare/unprepare/is_prepared operations

This adds prepare/unprepare/is_prepared functionality to the drivers for
the SI544 and SI514 chips, allowing the clock output to be disabled when
the clock is not in use.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>