mfd: rsmu: Split core code into separate module

mfd: rsmu: Split core code into separate module

Linking a file into two modules can have unintended side-effects
and produces a W=1 warning:

scripts/Makefile.build:236: drivers/mfd/Makefile: rsmu_core.o is added to multiple modules: rsmu-i2c rsmu-spi

Make this one a separate module instead.

Fixes: a1867f85e06e ("mfd: Add Renesas Synchronization Management Unit (SMU) support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240529094856.1869543-1-arnd@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>