driver: tee: Handle NULL pointer indication from client

TEE Client introduce a new capability "TEE_GEN_CAP_MEMREF_NULL"
to handle the support of the shared memory buffer with a NULL pointer.

This capability depends on TEE Capabilities and driver support.
Driver and TEE exchange capabilities at driver initialization.

Signed-off-by: Michael Whitfield <michael.whitfield@nxp.com>
Signed-off-by: Cedric Neveux <cedric.neveux@nxp.com>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (QEMU)
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>