pcw8256: update to the new major/minor numbers

Without this we panic with no tty on boot
pcw8256: comment out debugging for keyboard
pcw8256,README: Instructions for build/boot

Tested build and boot instructions to get a bootable kernel image. This is
a raw bootable image so you can boot it on joyce without owning any of the
Locoscript or CP/M 'start of day' discs