misc: pch_phub: Add UART_CLK quirk for Boston platform

The EG20T has 4 UART blocks. The clock source for the UART block is
configured to receive a clock from an external pin by default.

An internal 25MHz clock in the EG20T can also be used as a clock source
for the clock.

The MIPS based Boston platform ties the external clock pin down and relies
on the internal clock source for the UART to function.

Boston is based on device tree.

Add a quirk to allow Boston to be detected via device tree and set the
correct clock source for UART.

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