Support for "wide" ports

[ed-xmos]

1b ports are often scarce. It would be nice to support wide ports. I think this can be done using drive high/low if we can turn off the pull resistors. Unfortunately full control of the port is difficult due to some of the SETCs not working but I think there is a way.

The idea is as follows for pot reader:

• Make wide port input and sample + mask desired pin to get init value
• Change port to DRIVE_HIGH_WEAK_PULL_DOWN if value is low, or DRIVE_LOW_WEAK_PULL_UP if value is high
• Output a one or zero (opposite of init value) on the desired pin, all others will be weak pulled and wait to charge
• Change port to input (all Hi-Z)
• Wait for desired pin to cross threshold. pinsneq can be used with a port timer but it needs code to mask the value in SW as other pins may change at this time. So we need to repeat this case until we get the transition we want.

We will get a slight loss of accuracy due to having to mask on each pins.

[ed-xmos]

This has been done for potentiometer mode.

Rheostat mode not yet implemented but should be quite straightforward. Suggest we wait for demand for this.