machine: make I2C usable in the simulator

This fixes/improves a few issues with I2C support:

  * Validate I2C pins, so only pins that are supported by the hardware
    can be used (similar to how it's done with PWM).
  * Add address to Tx API (without it, the simulator can't really
    simulate I2C).
  * Add frequency when configuring. Not currently used, but might be
    useful in the future and adding it now avoids possibly breaking
    changes.

This is a breaking change, but since the simulator doesn't support I2C
yet that seems fine to me. (It does in my local changes, but those need
to be cleaned up before I can push them).

src/machine/machine_rp2040_simulator.go

@@ -94,3 +94,15 @@ var PWM7 = &timerType{
 		{GPIO15}, // channel B (1)
 	},
 }
+
+var I2C0 = &I2C{
+	Bus:     0,
+	PinsSCL: []Pin{GPIO1, GPIO5, GPIO9, GPIO13, GPIO17, GPIO21},
+	PinsSDA: []Pin{GPIO0, GPIO4, GPIO8, GPIO12, GPIO16, GPIO20},
+}
+
+var I2C1 = &I2C{
+	Bus:     0,
+	PinsSCL: []Pin{GPIO3, GPIO7, GPIO11, GPIO15, GPIO19, GPIO27},
+	PinsSDA: []Pin{GPIO2, GPIO6, GPIO10, GPIO14, GPIO18, GPIO26},
+}