We want to put the XO board clocks into the dt files, but we also
need to be backwards compatible with an older dtb. Add an API to
the common code to do this. This also makes a place for us to
handle the case when the RPM clock driver is enabled and we don't
want to register the fixed factor clock.

Cc: Georgi Djakov <georgi.djakov@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>