The UART's input clock rate can change at runtime but this is not
handled by the driver.

Add a clock_notifier callback that updates the divisors when the input
clock is updated. The serial8250_update_uartclk() is used to do so.
PRE_RATE_CHANGE and ABORT_RATE_CHANGE notifications are ignored, only
the POST_RATE_CHANGE is used. Not using PRE_RATE_CHANGE notification can
result in a few corrupted bytes during frequency transitions but, IMHO,
it can be acceptable in many use cases.

It has been tested on a DAVINCI/OMAP-L138 processor.
Signed-off-by: Bastien Curutchet <bastien.curutchet@bootlin.com>
Reviewed-by: Herve Codina <herve.codina@bootlin.com>
Link: https://lore.kernel.org/r/20240405120552.35991-1-bastien.curutchet@bootlin.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>