Move mutex definitions from compat.h to a dedicated mutex.h header, matching the Linux structure. U-Boot is single-threaded so mutexes are no-ops. The new header provides struct mutex and mutex_init/lock/unlock stubs. Update compat.h to include linux/mutex.h. Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com> Signed-off-by: Simon Glass <simon.glass@canonical.com>