powerpc/imc: Add documentation for IMC and trace-mode

Documentation for IMC (In-Memory Collection Counters) infrastructure
and trace-mode of IMC.

Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
[mpe: Convert to rst, minor rewording, make PMI example more concise]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191028100816.6270-1-mpe@ellerman.id.au