The mcpdm module found on omap4 and 5 needs pdmclk clock from
the pmic that may or may not be wired. Without this clock we
cannot read the registers for mcpdm at all. For the external
mcpdm clock to work, it needs to be muxed at the module level
for ti-sysc driver probe to mux it early enough for probe.

Let's set up a common file for it to make things a bit easier
to make l4 abe interconnect to probe with ti-sysc driver. Note
that this is not needed for omap5 as we can just update mcpdm
muxing in omap5-board-common.dtsi in later patches.

Cc: devicetree@vger.kernel.org
Cc: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>