pps: pps-gpio PPS ECHO implementation

This patch implements the PPS ECHO functionality for pps-gpio, that
sysfs claims is available already.

Configuration is done via device tree bindings.

No changes are made to userspace interfaces.

This patch was originally written by Lukas Senger as part of a masters
thesis project and modified for inclusion into the linux kernel by Tom
Burkart.

Link: http://lkml.kernel.org/r/20190324043305.6627-4-tom@aussec.com
Signed-off-by: Tom Burkart <tom@aussec.com>
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Signed-off-by: Lukas Senger <lukas@fridolin.com>
Cc: Philipp Zabel <philipp.zabel@gmail.com>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>