mmtimer: Remove the SGI SN2 mmtimer driver

mmtimer: Remove the SGI SN2 mmtimer driver

This driver supports direct system clock access on the ancient SGI SN2
IA64 systems, and implement the only non-builtin k_clock instance.
Remove it as any remaining IA64 altix user will be running just as old
distros anyway.

Dimitri Sivanich stated: "Since this is SN2 specific, this can be removed."

Note that this does not affect the never uv_mmtimer driver for x86-based
Altix systems.

[ tglx: Added comment to CLOCK_SGI_CYCLE ]

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Mike Travis <mike.travis@hpe.com>
Cc: Dimitri Sivanich <sivanich@hpe.com>
Link: http://lkml.kernel.org/r/20170526090311.3377-2-hch@lst.de