amba: Make use of bus_type functions

amba: Make use of bus_type functions

Instead of assigning the needed functions for each driver separately do it
only once in amba_bustype. Move the definition of the functions to their
proper place among the other callbacks used there. Note that the bus's
shutdown function might be called for unbound devices, too, so it needs
additional guarding.

This prepares getting rid of these callbacks in struct device_driver.

Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210126165835.687514-6-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>