Compile-testing the driver can result in a link failure
when CONFIG_SOC_BUS is disabled:

x86_64-linux-ld: drivers/soc/imx/soc-imx8m.o: in function `imx8_soc_init':
soc-imx8m.c:(.init.text+0x28d): undefined reference to `soc_device_register'

Select it from Kconfig, as we do from the other SoC drivers.

Link: https://lore.kernel.org/r/20200409075208.2824062-1-arnd@arndb.de
Fixes: fc40200e ("soc: imx: increase build coverage for imx8m soc driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>