MAINTAINERS: Change to my personal email address

I'm leaving SiFive in a bit less than two weeks, which means I'll be
losing my @sifive email address.  I don't have my new email address yet,
so I'm switching over to my personal address instead.

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