MIPS: oprofile: Add callgraph support

Stack unwinding is done by code examination. For kernelspace, the
already existing unwind function is utilized that uses kallsyms to
quickly find the beginning of functions. For userspace a new function
was added that examines code at and before the pc.

Signed-off-by: Daniel Kalmar <kalmard@homejinni.com>
Signed-off-by: Gergely Kis <gergely@homejinni.com>
Signed-off-by: Robert Richter <robert.richter@amd.com>