Allow changing pull-up mode on function pins

[davidcole1340]

At the moment it isn't possible to enable the pull-up resistors on pins that are in Function mode -- it is only enabled if the pin is in I2C function mode:

rp-hal/rp2040-hal/src/gpio/reg.rs

Lines 88 to 106 in 8e79f43

	Function(func) => {
	use DynFunction::*;
	fields.funcsel = match func {
	Xip => 0,
	Spi => 1,
	Uart => 2,
	I2C => 3,
	Pwm => 4,
	// SIO is 5, but isn't an alternate function but is instead for using the pin as GPIO
	Pio0 => 6,
	Pio1 => 7,
	Clock => 8,
	UsbAux => 9,
	};
	fields.inen = true;
	if func == I2C {
	fields.pue = true;
	}
	}

It would be nice to be able to enable/disable the pull-up resistors when the pin is in a different mode. My use case is enabling pull-up resistors with the pin in PIO function mode. The work-around is to take a disabled pin, convert it to pull-up input and then convert to a PIO pin. This is how i2c-pio does it:

https://github.com/rp-rs/i2c-pio-rs/blob/1a66fa53857dceb8e9ab1dc5be4fdda1b39107d4/src/lib.rs#L230-L252

[jannic]

Would it be best to allow all pad settings independent from the selected function?

After all, the peripheral and the pad configuration are completely separate on the rp2040 chip.

Of course it makes sense that a selected function also sets sensible defaults for the pad settings, like enabling pull-ups on I2C pins. However depending on the actual hardware connected to the pin, the user might want something different from the default.