Add basic cpu frequency control and temperature monitoring to imx6_anatop.

The temperature monitor device is enabled to sample the die temperature at
16hz.  The temperature is published via sysctl.  A callout routine at 10hz
monitors the temperature and throttles back the cpu if the temperature
goes over a user-settable throttle point (by default 10C less than the
critical high-point temperature for the chip).  The hardware is supposed
to be able to deliver an interrupt when the temperature exceeds a settable
limit, but the interrupt never arrives so for now a callout does the job.

At attach time we read the maximum cpu frequency the chip is allowed to run
at and the cpu is set to run at that speed.  It's reported at attach time.
A sysctl variable reports the current speed when queried.

New sysctl values:

  dev.imx6_anatop.0.cpu_mhz: 984
  dev.imx6_anatop.0.temperature: 37.9C
  dev.imx6_anatop.0.throttle_temperature: 95.0C

Steven Lawrance did the initial heavy lifting on this, but I changed
enough stuff that I'm the one to blame if anything breaks.

Submitted by:	Steven Lawrance <stl@koffein.net>
Add support for x2APIC virtualization assist in Intel VT-x.

The vlapic.ops handler 'enable_x2apic_mode' is called when the vlapic mode
is switched to x2APIC. The VT-x implementation of this handler turns off the
APIC-access virtualization and enables the x2APIC virtualization in the VMCS.

The x2APIC virtualization is done by allowing guest read access to a subset
of MSRs in the x2APIC range. In non-root operation the processor will satisfy
an 'rdmsr' access to these MSRs by reading from the virtual APIC page instead.

The guest is also given write access to TPR, EOI and SELF_IPI MSRs which
get special treatment in non-root operation. This is documented in the
Intel SDM section titled "Virtualizing MSR-Based APIC Accesses".

Enforce that APIC-write and APIC-access VM-exits are handled only if
APIC-access virtualization is enabled. The one exception to this is
SELF_IPI virtualization which may result in an APIC-write VM-exit.