tee: optee: add shared buffer registration functions

This change adds ops for shm_(un)register functions in tee interface.
Client application can use these functions to (un)register an own shared
buffer in OP-TEE address space. This allows zero copy data sharing between
Normal and Secure Worlds.

Please note that while those functions were added to optee code,
it does not report to userspace that those functions are available.
OP-TEE code does not set TEE_GEN_CAP_REG_MEM flag. This flag will be
enabled only after all other features of dynamic shared memory will be
implemented in subsequent patches. Of course user can ignore presence of
TEE_GEN_CAP_REG_MEM flag and try do call those functions. This is okay,
driver will register shared buffer in OP-TEE, but any attempts to use
this shared buffer will fail.

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