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>
Isn't this *static* RAM rather than *system* RAM? What about writing: *TrustZone protected on-chip SRAM* here, then just *SRAM* in the rest of the document?