x86: whack pmspcv from GENERIC

[mjg]

Summary

x86: whack pmspcv from GENERIC

The driver is enormous and rarely used.

      text      data       bss        dec         hex   filename
  23076646   1870505   4415872   29363023   0x1c00b4f   kernel.before
  20017433   1870305   4416000   26303738   0x1915cfa   kernel.after

Reviewed by:
Relnotes: people using the driver will need to add pmspcv_load="YES" to
their loader.conf
Sponsored by:   Rubicon Communications, LLC ("Netgate")

[emaste]

I think it should just be Relnotes: yes and mention what will need to be in the relnote in the commit message.