mp_hal_gpio_set_af will search for a given function and unit
and set the alternate function to the alternate function index
found.

py/mphal.h contains declarations for generic mp_hal_XXX functions, such
as stdio and delay/ticks, which ports should provide definitions for.  A
port will also provide mphalport.h with further HAL declarations.

Extracted GPIO clock enable logic into mp_hal_gpio_clock_enable
and called from anyplace which might need to use GPIO functions
on ports other than A-D.

Thanks to Dave Hylands for the patch.

Now all ports can use pybstdio.c to provide sys.stdin/stdout/stderr, so
long as they implement mp_hal_stdin_* and mp_hal_stdout_* functions.

This makes it easier to re-use readline.c and pyexec.c from stmhal in
other ports.

Addresses issue #968.

Addresses issue #921.