firmware: arm_ffa: Fetch the Rx/Tx buffer size using ffa_features()

firmware: arm_ffa: Fetch the Rx/Tx buffer size using ffa_features()

An endpoint can discover the minimum size, maximum size and alignment
boundary for the Rx/Tx buffers by passing the function ID of the
FFA_RXTX_MAP ABI as input in the FFA_FEATURES interface. The maximum
size is an optional field and a value of 0 means that the partition
manager or the hypervisor does not enforce a maximum size.

Use the discovery mechanism and remove the hardcoded 4kB buffer size.
If the discovery fails, it still defaults to 4kB.

Message-Id: <20240820-ffa_v1-2-v2-7-18c0c5f3c65e@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>