.gitmodules: move u-boot mirrors to qemu-project-mirrors

.gitmodules: move u-boot mirrors to qemu-project-mirrors

To continue our GitLab Open Source Program license we need to pass an
automated license check for all repos under qemu-project. While U-Boot
is clearly GPLv2 rather than fight with the automated validation
script just move the mirror across to a separate project.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20250908141911.2546063-1-alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit a11d1847d5ef8a7db58e6d4e44f36fec708f0981)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>