serial: pch_uart: Add support for reading clock-frequency from DT

The MIPS based Boston platform provides a 25MHz clock to the UART.

Enable the driver for MIPS and add support in the driver to read
the frequency from device tree.

Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>