MAINTAINERS: Change to my personal email address

I left SiFive a bit more than three months ago, and while I sent out a message
saying I was going to updated my email address I neclected to actually do so.
I'm moving to my personal email address to avoid the need to do this again.

gcc/ChangeLog

2020-02-20  Palmer Dabbelt  <palmer@sifive.com>

        * MAINTAINERS: Change palmer@sifive.com to palmer@dabbelt.com.