serial: 8250_dw: clock rate handling for all ACPI platforms

This replaces the Baytrail specific custom set_termios hook
with a more generic one where the clock framework is used to
set the rate. The method also doesn't need to be limited to
just Baytrail, so it's used with all ACPI platforms.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>