staging: mt7621-pci: dt-bindings: update bindings doc removing sysctls registers

The sysctl register are already claimed by palmbus, so
pci fails to claim it. Device tree has been updated to not
use it at all with pcie. Update bindings documentation.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Tested-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>