serial: 8250_dw: Introduce IO accessors to extended registers

There are several extended (in comparison to the traditional 16550)
registers are present in Synopsys DesignWare UART. All of them
are 32-bit ones.

Introduce helpers to simplify access to them and convert existing users.

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