MAINTAINERS: update the 'T:' entry for xtensa

All development activity for xtensa architecture have been routed
through the github.com/jcmvbkbc/linux-xtensa tree for the last few
years. Update the 'T:' entry in the MAINTAINERS file to reflect that.

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>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>