tee: optee: don't enumerate services if there ain't any

Change optee driver service enumeration to not enumerate (and
allocate a zero sized shared memory buffer) when OP-TEE
reports that there is no service to enumerate.

This change fixes an existing issue that occurs when the such zero
sized shared memory buffer allocated from malloc() has a physical
address of offset 0 of a physical 4kB page. In such case, OP-TEE
secure world refuses to register the zero-sized shared memory
area and makes U-Boot optee service enumeration to fail.

Fixes: 94ccfb78 ("drivers: tee: optee: discover OP-TEE services")
Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>