ACPI / LPSS: support for fractional divider clock

This creates fractional divider type clock for the ones that
have it. It is needed by the UART driver as the clock rate must
accommodate to the requested baud rate.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>