firmware: arm_scmi: Move scmi bus init and exit calls into the driver

firmware: arm_scmi: Move scmi bus init and exit calls into the driver

In preparation to enable building scmi as a single module, let us move
the scmi bus {de-,}initialisation call into the driver.

The main reason for this is to keep it simple instead of maintaining
it as separate modules and dealing with all possible initcall races
and deferred probe handling. We can move it as separate modules if
needed in future.

Link: https://lore.kernel.org/r/20200907195046.56615-3-sudeep.holla@arm.com
Tested-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>