siox: constify the struct device_type usage

Since commit aed65af1 ("drivers: make device_type const"), the driver
core can properly handle constant struct device_type. Move the
siox_device_type and siox_master_type variables to be constant structures
as well, placing it into read-only memory which can not be modified at
runtime.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: "Ricardo B. Marliere" <ricardo@marliere.net>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Thorsten Scherer <t.scherer@eckelmann.de>
Link: https://lore.kernel.org/r/20240219-device_cleanup-siox-v1-1-eb32ca2b0113@marliere.net


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>