serial: 8250_of: add support for an optional bus clock

The SpacemiT UART requires a bus clock to be enabled, in addition to
it's "normal" core clock.  Look up the optional bus clock by name,
and if that's found, look up the core clock using the name "core".

Supplying a bus clock is optional.  If no bus clock is needed, the
the first/only clock is used for the core clock.

Signed-off-by: Alex Elder <elder@riscstar.com>
Link: https://lore.kernel.org/r/20250409192213.1130181-3-elder@riscstar.com


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