bus: Allow reserving hardware interfaces with [name] syntax

The special name "_" is treated as the bus name.

klippy/extras/bus.py

@@ -31,6 +31,13 @@ def resolve_bus_name(mcu, param, bus):
     if reserve_pins is not None:
         pin_resolver = ppins.get_pin_resolver(mcu_name)
         for pin in reserve_pins.split(','):
+            if pin.startswith('[') and pin.endswith(']'):
+                interface = pin[1:-1]
+                if interface == '_':
+                    interface = bus
+                pin_resolver.reserve_interface(interface, "%s[%s]" % (
+                    param, bus))
+            else:
                 pin_resolver.reserve_pin(pin, bus)
     return bus