Die arrangement and pins documentation change.

[d-m-bailey]

[caravel/docs/source/supplementary-figures.rst](https://github.com/efabless/caravel/blob/main/docs/source/_static/die_pads.svg)

The diagram for Die arrangement and pins indicates that mprj_io[0:7] have fixed defaults but the actual scripts/gen_gpio_defaults.py only has fixed values for mprj_io[0:4].