reset: sti: softreset: Remove obsolete platforms from dt binding doc.

STiH415/6 SoC support is being removed from the kernel.
This patch updates the sti dt softreset bindings and
removes references to these obsolete platforms.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>