firmware: arm_ffa: Set reserved/MBZ fields to zero in the memory descriptors

The transmit buffers allocated by the driver can be used to transmit data
by any messages/commands needing the buffer. However, it is not guaranteed
to have been zero-ed before every new transmission and hence it will just
contain residual value from the previous transmission. There are several
reserved fields in the memory descriptors that must be zero(MBZ). The
receiver can reject the transmission if any such MBZ fields are non-zero.

While we can set the whole page to zero, it is not optimal as most of the
fields get initialised to the value required for the current transmission.

So, just set the reserved/MBZ fields to zero in the memory descriptors
explicitly to honour the requirement and keep the receiver happy.

Fixes: cc2195fe

 ("firmware: arm_ffa: Add support for MEM_* interfaces")
Reported-by: Marc Bonnici <marc.bonnici@arm.com>
Link: https://lore.kernel.org/r/20230503131252.12585-1-sudeep.holla@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>