optee: add FF-A capability OPTEE_FFA_SEC_CAP_ARG_OFFSET

Adds the secure capability OPTEE_FFA_SEC_CAP_ARG_OFFSET to indicate that
OP-TEE with FF-A can support an argument struct at a non-zero offset into
a passed shared memory object.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>