trusted-firmware-a: add support for SPD (Secure Payload Dispatcher) services

Some platforms use Secure Payload Dispatcher - allow selecting one with TFA_SPD.

Official SPD description:
/*******************************************************************************
 * This is the Secure Payload Dispatcher (SPD). The dispatcher is meant to be a
 * plug-in component to the Secure Monitor, registered as a runtime service. The
 * SPD is expected to be a functional extension of the Secure Payload (SP) that
 * executes in Secure EL1. The Secure Monitor will delegate all SMCs targeting
 * the Trusted OS/Applications range to the dispatcher. The SPD will either
 * handle the request locally or delegate it to the Secure Payload. It is also
 * responsible for initialising and maintaining communication with the SP.
 ******************************************************************************/

Clean other EXTRA_OEMAKE to remove leading space not needed with +=

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Reviewed-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>