linuxkpi: Define raw_spinlock_t in <linux/spinlock.h>

For now, it is synonymous to spinlock_t. The DRM generic code uses the
struct raw_spinlock and not raw_spinlock_t, that's why the
definition is a struct embedding a struct mtx, compared to
spinlock_t which is a simpler typedef.

The DRM generic code started to use it in Linux 6.10.

Reviewed by: bz, christos
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54489