Enhancement: Bootstrap Voltage Select

[daveythacher]

From section 2.19.4 of datasheet:

Both the output signal level and acceptable input signal level at the pad are determined by the digital IO supply (IOVDD).
IOVDD can be any nominal voltage between 1.8V and 3.3V, but to meet specification when powered at 1.8V, the pad
input thresholds must be adjusted by writing a 1 to the pad VOLTAGE_SELECT registers. By default the pad input thresholds
are valid for an IOVDD voltage between 2.5V and 3.3V. Using a voltage of 1.8V with the default input thresholds is a safe
operating mode, though it will result in input thresholds that don’t meet specification.

Context: https://forums.raspberrypi.com/viewtopic.php?t=328887

Would be nice to use a GPIO during bootloader to set this. A simple pull-down vs pull-up should not compromise the pin's operation. Not critical but would be nice on next version of RP2040, whenever that may be. Should be possible to ensure backwards compatibility.

Note this may be considered pointless in most applications. This could be fixed few ways if this becomes problematic, however these work arounds could increase cost.