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>

MAINTAINERS

@@ -18777,7 +18777,7 @@ M:	Palmer Dabbelt <palmer@dabbelt.com>
 M:	Paul Walmsley <paul.walmsley@sifive.com>
 L:	linux-riscv@lists.infradead.org
 S:	Supported
-T:	git git://github.com/sifive/riscv-linux.git
+T:	git https://github.com/sifive/riscv-linux.git
 N:	sifive
 K:	[^@]sifive