stm32f0: systick_blink: Robustness and correctness fixes

Update documentation and comments in code to reflect reality.
Ensure that GPIO pin output speed is actually set, to ensure that 48Mhz sysclk
output is functional.
Actually set AF, instead of relying on reset values.
Replace the systick code, the core of this example, with some code that has
less traps and surprises.  Instead of trying to get a direct interrupt x times
per second, and reguarly running into problems with the 24 bit counter limit,
use a method that triggers an interrupt every x ms instead.  Tested MCO and
blink rates with a logic analyser, properly verified working now :)

examples/stm32/f0/stm32f0-discovery/systick_blink/README.md

@@ -10,4 +10,4 @@ to debug the PLL clock setup by scope.
 
 | Port  | Function | Description              |
 | ----- | -------- | ------------------------ |
-| `PA9` | `(MCO)`  | Internal reference clock |
+| `PA8` | `(MCO)`  | Internal reference clock |