mfd: ocelot-spi: Add missing MODULE_DEVICE_TABLE

This patch adds missing MODULE_DEVICE_TABLE definition which generates
correct modalias for automatic loading of this driver when it is built
as an external module.

Fixes: f3e89362 ("mfd: ocelot: Add support for the vsc7512 chip via spi")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220922103703.1731266-1-yangyingliang@huawei.com