serial: 8250_lpss: add fractional divisor support

For Synopsys DesignWare 8250 uart which version >= 4.00a, there's a
valid divisor latch fraction register.

Now the preparation is done, it's easy to add the feature support.
This patch firstly tries to get the fractional divisor width during
probe, then setups specific get_divisor() and set_divisor() hook.

Among other changes the FIFO size is now retrieved from the hardware.

Signed-off-by: Andy Shevchenko <[email protected]>
Reviewed-by: Heikki Krogerus <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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