firmware: arm_ffa: Add support for FFA_YIELD in direct messaging

firmware: arm_ffa: Add support for FFA_YIELD in direct messaging

Successful completion of both direct messaging function can be indicated
through an invocation of FFA_YIELD or GGA_INTERRUPT by the callee.

FFA_INTERRUPT indicates that the direct request was interrupted and must
be resumed through the FFA_RUN interface which is already done in the
driver.

FFA_YIELD indicates that the receiver endpoint has transitioned to the
blocked runtime state and must be resumed through the FFA_RUN interface.
However, the way receiver endpoint gets unblocked is implementation
defined. So, the driver just sleeps for 1 - 2ms and issues FFA_RUN. It
can return to the caller with FFA_YIELD is the receiver endpoint is still
blocked.

Message-Id: <20240820-ffa_v1-2-v2-6-18c0c5f3c65e@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>