bus: ti-sysc: Add quirk handling for external optional functional clock

We cannot access mcpdm registers at all unless there is an optional pdmclk
configured. As this is currently only needed for mcpdm, let's check for
mcpdm in sysc_get_clocks(). If it turns out to be needed for other modules
too, we can add more flags to the quirks table for this.

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