qseecom: Add support to issue secure service cmd.

 - Add a generic way to issue commands to secure services running
on the secure domain (TrustZone). This is implemented
in the qseecom_svc_cmd() API.

 - This API currently supports RPMB commands to the respective
RPMB servcie running on TZ.

Change-Id: I38148b7fd2cee72146368dd1b2f902aa234847b7
Signed-off-by: Hariprasad Dhalinarasimha <hnamgund@codeaurora.org>