freedreno/registers: Add prefix="variant"

freedreno/registers: Add prefix="variant"

To merge a7xx and a6xx regs, using variant property to manage the
differences, we'll want regs/etc to be named according to the first
generation it is use rather than the domain name.  Add a new prefix
type to accomplish this.  By default, if no variant property, things
will still be named based on domain (ie. REG_A6XX_...), and things
that have variant="A6XX" will also end up as they currently are
(since the chip enum matches domain name), but things that have
variant="A7XX" will end up as REG_A7XX_...

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21846>