firmware: arm_ffa: Helper to check if a partition can receive REQUEST2 messages

Add a helper that allows FF-A drivers to check if the partition can
receive the direct requests via the FFA_MSG_SEND_DIRECT_REQ2 ABI.

Tested-by: Viresh Kumar <viresh.kumar@linaro.org>
Message-Id: <20250217-ffa_updates-v3-9-bd1d9de615e7@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>