Emulate the AMD MSR_LS_CFG MSR used for various Ryzen errata.

Writes are ignored and reads always return zero.

Submitted by: José Albornoz <jojo@eljojo.net> (write-only version)
Reviewed by: Patrick Mooney, cem
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D19506