ipq806x: chromium: Support Chromium "crossystem" GPIO tooling

ipq806x: chromium: Support Chromium "crossystem" GPIO tooling

ChromiumOS's vboot_reference tooling [1] provides convenient access to
various firmware and hardware details via its `crossystem` tool.
crossystem currently:
(1) relies on the v1 GPIO cdev API to read GPIOs; and
(2) expects gpio-line-names properties.

Enable the kernel config, and document a few pins for OnHub devices.

I only go so far as to pull two relevant names out of the vendor device
tree. Others could perhaps be backfilled if the info is available and
useful.

[1] https://chromium.googlesource.com/chromiumos/platform/vboot_reference/+/HEAD/README

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16014
Signed-off-by: Robert Marko <robimarko@gmail.com>