dt-bindings: serial: 8250: add SpacemiT K3 UART compatible

dt-bindings: serial: 8250: add SpacemiT K3 UART compatible

The SpacemiT K3 UART controller is compatible with the Intel XScale UART.
Add K3 UART binding and allow describing it with a fixed clock-frequency
for now.

The clocks and clock-names properties will be made mandatory in a future
patch, once the K3 clock driver and device tree are merged.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Guodong Xu <guodong@riscstar.com>
Link: https://patch.msgid.link/20251222-k3-basic-dt-v2-5-3af3f3cd0f8a@riscstar.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>