serial: 8250_dw: remove leftover definitions

The clocks are managed through clk-fractional-divider.c module, and thus CLK
framework takes care about it. Remove letfovers from this driver.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>