bus: ti-sysc: Add separate functions for handling clocks

At least McPDM module depends on an external optional clock to be
usable. To make handling of the McPDM clock easier in the following
patches, let's add separate functions for handling the main clocks
and the optional clocks.

Let's also add error handling to shut down already enabled clocks
while at it.

Signed-off-by: Tony Lindgren <tony@atomide.com>