[LLVM] Update RISCV maintainers

Add Philip Reames and myself as maintainers. I think between the two
of us we do a lot of the patch reviews. I didn't put specific areas
and I'm happy to add more names.

Author: topperc

llvm/Maintainers.md

@@ -250,6 +250,10 @@ [email protected] (email), [chenzheng1030](https://github.com/chenzheng1030) (
 
 Alex Bradbury \
 [email protected] (email), [asb](https://github.com/asb) (GitHub)
+Craig Topper \
+[email protected] (email), [topperc](https://github.com/topperc) (GitHub)
+Philip Reames \
+[email protected] (email), [preames](https://github.com/preames) (GitHub)
 
 #### Sparc backend