hw/gpio/pl061: Document a shortcoming in our implementation

hw/gpio/pl061: Document a shortcoming in our implementation

The Luminary PL061s in the Stellaris LM3S9695 don't all have the same
reset value for GPIOPUR.  We can get away with not letting the board
configure the PUR reset value because we don't actually wire anything
up to the lines which should reset to pull-up.  Add a comment noting
this omission.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>