power: reset: st-poweroff: Remove obsolete platforms.

This patch removes support for STiH415/6 SoC's from the
st-poweroff driver, as support for these platforms is
being removed from the kernel.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Cc: <linux-pm@vger.kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>