tile: vdso: use raw_read_seqcount_begin() in vdso

Previously we were using read_seqcount_begin(), which works fine until
lockdep is enabled in the kernel, at which point lockdep locking shows
up in the vdso and userspace will take a GPV accessing a kernel-only
SPR when calling gettimeofday() etc.

Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>