Documentation: ARM: EXYNOS: Describe boot loaders interface

Various boot loaders for Exynos based boards use certain memory
addresses during booting for different purposes. Mostly this is one of
following :
1. as a CPU boot address,
2. for storing magic cookie related to low power mode (AFTR, sleep).

The document, based solely on kernel source code, tries to group the
information scattered over different files. This would help in the
future when adding support for new SoC or when extending features
related to low power modes.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>