MAINTAINERS: git://github.com -> https://github.com for sifive

Github deprecated the git:// links about a year ago, so let's move to
the https:// URLs instead.

Reported-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://github.blog/2021-09-01-improving-git-protocol-security-github/


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