MAINTAINERS: Add Paul as a RISC-V maintainer

The RISC-V port has grown significantly over the past year.  Paul's been
helping out for a while ago.  We agreed in person that he'd take over
collecting the patches and submitting the PRs, but it looks like I
forgot to make it official.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>