ARM: dts: imx6: Fix PCIe reset GPIO polarity on Toradex Apalis Ixora

Adding reset-gpio-active-high boolean DT binding property, which we need to
make PCIe working on Apalis SoMs and not break old DTBs. While at it, I've
fixed comment and GPIO polarity.

On Apalis SoMs the GPIO1_IO28 used to PCIe reset is not connected directly
to PERST# PCIe signal, but it's ORed with RESETBMCU coming off the PMIC,
and thus is inverted, active-high.

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>