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

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

Acked-by: William Zhang <william.zhang@broadcom.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
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: Florian Fainelli <f.fainelli@gmail.com>