Fix NULL FUNCSEL handling on Raspberry Pi 5

[lukeburong]

Fixes #142

This introduces a new variant to gpio::Mode called Null (though something like Disabled may be more appropriate - I just stuck with the datasheet).

After implementing this, the example code from #142 gives the following results on a Raspberry Pi 5:

pinctrl 25 before:

25: no    pn | -- // GPIO25 = none

pinctrl 25 after:

25: ip    pn | lo // GPIO25 = input

And with set_reset_on_drop(true), the pin mode is restored correctly on drop:

pinctrl 25 before:

25: no    pn | -- // GPIO25 = none

pinctrl 25 after:

25: no    pn | -- // GPIO25 = none

The new variant shouldn't affect anything not using the RP1 (much like Alt6 - Alt8 which are also only used in rp1.rs)