Chrono compilation errors on AVR

[jputcu]

When using chrono on AVR I get the following error:

etl/private/chrono/duration.h:318:62: error: 'nano' is not a member of 'etl'
  318 |     using nanoseconds  = etl::chrono::duration<int64_t, etl::nano>;

Because int is 16-bit the following, in ratio.h, is not provided:

 #if (INT_MAX >= INT32_MAX)
    typedef ratio<1, 1000000000>                nano;
    typedef ratio<1, 1000000>                   micro;
 #endif

Probably, this cross compiler check also needs to go in duration.h to exclude nanoseconds and microseconds.