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Some platforms, namely AMD Picasso, use non standard uart clocks (48M),
witch makes it impossible to use with earlycon.

Let the user select its own frequency.
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20221123-serial-clk-v3-1-49c516980ae0@chromium.org

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>