extmod/modmachine: Factor ports' machine module dict to common code.

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>

ports/esp32/esp32_common.cmake

@@ -69,7 +69,6 @@ list(APPEND MICROPY_SOURCE_PORT
     machine_touchpad.c
     machine_dac.c
     machine_i2c.c
-    modmachine.c
     network_common.c
     network_lan.c
     network_ppp.c