ARM: Expose some CPU control registers via sysfs

This creates sysfs files under /sys/devices/system/cpu/cpuN
exposing the values of the control register, auxiliary control
register, and L2 cache auxiliary control register.  Writing to
the files allows setting the value of bits which are safe to
change at any time.

Signed-off-by: Mans Rullgard <mans@mansr.com>