tee: optee: add optee-specific shared pool implementation

This is simple pool that uses kernel page allocator. This pool can be
used in case OP-TEE supports dynamic shared memory.

Signed-off-by: Volodymyr Babchuk <vlad.babchuk@gmail.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>