x86/kvmclock: convert to SPDX identifiers

Update the verbose license text with the matching SPDX
license identifier.

Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
[Changed deprecated GPL-2.0+ to GPL-2.0-or-later. - Radim]
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>