firmware: arm_ffa: Emit modalias for FF-A devices

In order to enable libkmod lookups for FF-A device objects to their
corresponding module, add 'modalias' to the base attribute of FF-A
devices.

Tested-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>
Link: https://lore.kernel.org/r/20231005175640.379631-1-sudeep.holla@arm.com


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>