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/rp2/CMakeLists.txt

@@ -125,7 +125,6 @@ set(MICROPY_SOURCE_PORT
     machine_spi.c
     machine_timer.c
     main.c
-    modmachine.c
     modrp2.c
     mphalport.c
     mpnetworkport.c
@@ -153,7 +152,6 @@ set(MICROPY_SOURCE_QSTR
     ${MICROPY_PORT_DIR}/machine_timer.c
     ${MICROPY_PORT_DIR}/machine_uart.c
     ${MICROPY_PORT_DIR}/machine_wdt.c
-    ${MICROPY_PORT_DIR}/modmachine.c
     ${MICROPY_PORT_DIR}/modrp2.c
     ${MICROPY_PORT_DIR}/modos.c
     ${MICROPY_PORT_DIR}/rp2_flash.c