CREDITS: Add -next to Stephen Rothwell's entry

Stephen retired and stepped back from -next maintainership, update his
entry in CREDITS to recognise his 18 years of hard work making it what
it is today and all the impact it's had on our development process.

Also update to his current GnuPG key while we're here.

Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>