Moved from interrupts.c to cpu.c and made into a weak function to
allow vendor specific override

Vendor specific CPU reset (like the AMD SC520 MMCR reset) can now be
added to the vendor specific code without the need to remember to
#undef usage of the generic method and if you forget to include your
custom reset method, you will always get the default.

Signed-off-by: Graeme Russ <graeme.russ@gmail.com>