Add documentation for the powerpc memtrace debugfs files

CONFIG_PPC_MEMTRACE must be set to use this feature. This can only be
used on powernv platforms.

Signed-off-by: Rashmica Gupta <rashmica.g@gmail.com>
[mpe: Update dates and kernel versions, mention size is in bytes]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>