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>
Add a header maybe?
```
  Type      Sections
+---------+----------------+
|         | text           |
|         | rodata         |
|         | data           |
| unpaged | bss            |
```