pager documentation

Updates the pager section of the OP-TEE design document with a
description of the pager.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
You do not explain how you put the code and data into the appropriate sections.
It would probably not make much sense to give many details, but I think it is worth mentioning the big picture, basically: use compiler flags + the linker `--gc-sections` to eliminate symbols that are outside a dependency graph, then use readelf and awk scripts to generate bits of the final linker script. Well, if I understand correctly ;)