firmware: arm_ffa: Declare ffa_bus_type structure in the header

smatch reports:
drivers/firmware/arm_ffa/bus.c:108:17: warning:
	symbol 'ffa_bus_type' was not declared. Should it be static?

ffa_bus_type is exported to be useful in the FF-A driver. So this
warning is not correct. However, declaring the ffa_bus_type structure
in the header like many other bus_types do already removes this warning.
So let us just do the same and get rid of the warning.

Link: https://lore.kernel.org/r/20231024105715.2369638-1-sudeep.holla@arm.com


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