__GPIOI_CLK_ENABLE is defined in hal/l4/inc/Legacy/stm32_hal_legacy.h
as __HAL_RCC_GPIOI_CLK_ENABLE, and that latter macro is not defined
anywhere else (because the L4 does not have port GPIOI).  So the test
for GPIOI is needed, along with the test for the CLK_ENABLE macro.

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.