x86/uv: provide a System Activity Indicator driver

Impact: start per CPU heartbeat LED timers on SGI UV systems

The SGI UV system has no LEDS but uses one of the system controller
regs to indicate the online internal state of the cpu.  There is a
heartbeat bit indicating that the cpu is responding to interrupts,
and an idle bit indicating whether the cpu is idle when the heartbeat
interrupt occurs.  The current period is one second.

When a cpu panics, an error code is written by BIOS to this same reg.

This patchset provides the following:

  * x86_64: Add base functionality for writing to the specific SCIR's
    for each cpu.

  * heartbeat: Invert "heartbeat" bit to indicate the cpu is
    "interruptible".  If the current thread is the idle thread,
    then indicate system is "idle".

  * if hotplug enabled, all bits are set (0xff) when the cpu is disabled.

Signed-off-by: Mike Travis <travis@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

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