Name already in use

This is a code factoring to have the dict for the machine module in one
location, and all the ports use that same dict.  The machine.soft_reset()
function implementation is also factored because it's the same for all
ports that did already implement it.  Eventually more functions/bindings
can be factored.

All ports remain functionally the same, except:
- cc3200 port: gains soft_reset, mem8, mem16, mem32, Signal; loses POWER_ON
  (which was a legacy constant, replaced long ago by PWRON_RESET)
- nrf port: gains Signal
- qemu-arm port: gains soft_reset
- unix port: gains soft_reset
- zephyr port: gains soft_reset, mem8, mem16, mem32

Signed-off-by: Damien George <damien@micropython.org>