Porting PR micropython#8040 of @hoihu for SAMD, following the commit
5873390.

---

Issue:

This PR unveils an issue of the port and is made to have a working
example for easily reproducing the fault.
USB locks up almost immediately if charaters at the input arrive too
fast, when they are echoed. That happens for instance in REPL.
This issue is present in the actual master branch as well, but requires
much higher input symbol frequency and is hard to reproce.

Addition:

Temporary add  a function mp_hal_time_ns() to please the linker when
built with DEBUG=1.

- Support executing .mpy files.
- Fix UART REPL config error
- Enable Pull on UART RX.
  Now, that UART works, it avoids a crosstalk when the input not wired.
- Do not use UART for REPL when not initialized.
  The standard behavior enables REPL on UART at boot time. That can be
  disabled using machine.uart_deinit() and re-enabled again with
  machine.uart_init(). uart_deinit() disabled until now only the gpio
  port configuration, but data was still sent to the UART module. This
  commit stops using UART after uart_deinit() was called.
- use -Og for DEBUG builds, whuch creates a smaller image.
- Skip mp_hal_time_ns() for regular builds.
- Set the CPU freq to 120MHz for SAM51 devices.

This is temporary and does not fit to the SAMD21 build with DEBUG enabled.