x86/cpu/cyrix: Add alternative Device ID of Geode GX1 SoC

x86/cpu/cyrix: Add alternative Device ID of Geode GX1 SoC

A SoC variant of Geode GX1, notably NSC branded SC1100, seems to
report an inverted Device ID in its DIR0 configuration register,
specifically 0xb instead of the expected 0x4.

Catch this presumably quirky version so it's properly recognized
as GX1 and has its cache switched to write-back mode, which provides
a significant performance boost in most workloads.

SC1100's datasheet "Geode™ SC1100 Information Appliance On a Chip",
states in section 1.1.7.1 "Device ID" that device identification
values are specified in SC1100's device errata. These, however,
seem to not have been publicly released.

Wading through a number of boot logs and /proc/cpuinfo dumps found on
pastebin and blogs, this patch should mostly be relevant for a number
of now admittedly aging Soekris NET4801 and PC Engines WRAP devices,
the latter being the platform this issue was discovered on.
Performance impact was verified using "openssl speed", with
write-back caching scaling throughput between -3% and +41%.

Signed-off-by: Christian Sünkenberg <christian.suenkenberg@student.kit.edu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1496596719.26725.14.camel@student.kit.edu
Signed-off-by: Ingo Molnar <mingo@kernel.org>