pinctrl: st: Remove obsolete platforms from pinctrl-st dt doc

STiH415/6 SoC support is being removed from the kernel.
This patch updates the ST pinctrl dt doc and removes
references to these obsolete platforms. It also updates
the dt example to the currently supported STiH407
platform.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Cc: <linus.walleij@linaro.org>
Cc: <robh+dt@kernel.org>
Cc: <linux-gpio@vger.kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>