target-alpha: use HTTPS git URL for palcode

When you clone the repository without previous commit history, 'git://'
doesn't protect from man-in-the-middle attacks.  HTTPS is more secure
since the client verifies the server certificate.

Cc: Richard Henderson <rth@twiddle.net>
Suggested-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20181108111531.30671-8-stefanha@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

pc-bios/README

@@ -36,7 +36,7 @@
 	1af4:1000 -> pxe-virtio.rom
 
 - The sources for the Alpha palcode image is available from:
-  git://github.com/rth7680/qemu-palcode.git
+  https://github.com/rth7680/qemu-palcode.git
 
 - The u-boot binary for e500 comes from the upstream denx u-boot project where
   it was compiled using the qemu-ppce500 target.