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_generic_peripherals.go

@@ -10,5 +10,4 @@ var (
 	UART1 = hardwareUART1
 	SPI0  = &SPI{0}
 	SPI1  = &SPI{1}
-	I2C0  = &I2C{0}
 )