gpio: ftgpio: Support debounce timer

The FTGPIO010 has a debounce timer or rather prescaler that
will affect interrupts fireing off the block. We can support
this to get proper debounce on e.g. keypresses.

Since the same prescaler is used across all GPIO lines of
the silicon block, we need to bail out if the prescaler is
already set and in use by another line.

If the prescaler is already set to what we need, fine, we
reuse it. This happens more often than not when the same
debounce time is set for several GPIO keys, so we support
that usecase easily with this code.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>