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>

MAINTAINERS

@@ -13717,6 +13717,7 @@ F:	drivers/mtd/nand/raw/r852.c
 F:	drivers/mtd/nand/raw/r852.h
 
 RISC-V ARCHITECTURE
+M:	Paul Walmsley <paul.walmsley@sifive.com>
 M:	Palmer Dabbelt <palmer@sifive.com>
 M:	Albert Ou <aou@eecs.berkeley.edu>
 L:	linux-riscv@lists.infradead.org