Update GPIO pullup/pulldown code to support Pi4
hdr.BCM2835: added 4 equates for offsets to pullup/down control registers that are new with BCM2711. This change is safe on its own but is required by the changes to s.GPIO.
s.GPIO: the way that pullups and pulldowns are controlled in the BCM2711 is completely different from previous models. It's no longer possible to change settings for one line and leave the others unchanged; a whole register at a time must be rewritten. There's more than one way to do it, of course. I opted to update the shadow copies of the pull settings from the hardware and then modify them from there.
Tested on Pi4 and Pi3B+.