@Linus Walleij,
From a technical standpoint, this GPIO driver is no different from
gpio-mxc, gpio-omap, or gpio-rda.

Yes it is different. The example you list are all silicon GPIO blocks.
This driver is not for silicon, is a protocol spoken over rpmsg.  The
concept of two CPUs connected by rpmsg in a SoC is used by a number of
vendors. Look in Documentation/devicetree/bindings/remoteproc you see:

amlogic, fsl, ingenic, mtk, qcom, reneses, st, ti, wkup and xlnx.

We want one generic protocol/implementation of GPIO over rpmsg which
all these vendors will use. That means we have one GPIO driver, not 10
drivers to maintain for the next 10-20 years. It also means those 10
vendors have 1/10 of a driver they need to maintain for the next 10-20
years. And likely less bugs to deal with, since the driver is more
heavily tested by 10 vendors, etc.

     Andrew