dt-bindings: serial: 8250: support an optional second clock

dt-bindings: serial: 8250: support an optional second clock

The SpacemiT UART driver requires a bus clock to be enabled in addition
to the primary function clock.  Add the option to specify two clocks
for an 8250-compatible UART, named "core" and "bus".  If both are needed,
require them to be named.

Signed-off-by: Alex Elder <elder@riscstar.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250409192213.1130181-2-elder@riscstar.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>