firmware: arm_scmi: Introduce setup_shmem_iomap

To get the address of shmem could be generalized by introducing
setup_shmem_iomap. Then the duplicated code in mailbox.c, optee.c
and smc.c could be dropped.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
[ Cristian: use OF __free and make use of the new helper also in smc.c ]
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Tested-by: Peng Fan <peng.fan@nxp.com>  #i.MX95 19x19 EVK
Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>
Message-Id: <20240812173340.3912830-3-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>