gpio: ftgpio: Support optional silicon clock

The GPIO silicon is clocked with a PCLK (peripheral clock)
on all systems, however not all platforms model it and include
it in e.g. the device tree, so add clock handling but make it
optional so we bail out safely if it is e.g. always on.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>