Documentation: arm: define DT cpu capacity-dmips-mhz bindings

ARM systems may be configured to have cpus with different power/performance
characteristics within the same chip. In this case, additional information
has to be made available to the kernel (the scheduler in particular) for it
to be aware of such differences and take decisions accordingly.

Therefore, this patch aims at standardizing cpu capacities device tree
bindings for ARM platforms. Bindings define cpu capacity-dmips-mhz
parameter, to allow operating systems to retrieve such information from
the device tree and initialize related kernel structures, paving the way
for common code in the kernel to deal with heterogeneity.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Olof Johansson <olof@lixom.net>
Cc: Gregory CLEMENT <gregory.clement@free-electrons.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>