MAINTAINERS: Add myself as a maintainer for SiFive's drivers

There aren't actually any files in the tree that match these patterns
right now, but we've just started submitting our drivers so I thought it
would be good to make sure there's at least someone at SiFive who's
listed as maintaining them.  I'm leaving the RISC-V lists on here
because:

* As of today, all the RISC-V ASICs that people can actually buy are
  from SiFive -- though hopefully there'll be more soon!
* The RTL for many of our devices is open source, so I anticipate these
  devices might make they way chips from other vendors.
* We may standardize some of these devices as part of a RISC-V
  specification at some point in the future.

I'm a bit swamped right now so I might not be the most active maintainer
of these drivers, but I think it'd be good to make sure someone who has
hardware access gets CC'd on updates to our drivers just as a sanity
check.  Hopefully that's an OK way to handle this.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>