On powerpc, it is possible to compile test both the new apple (arm) and
old pasemi (powerpc) drivers for the i2c hardware at the same time,
which leads to a warning about linking the same object file twice:

scripts/Makefile.build:244: drivers/i2c/busses/Makefile: i2c-pasemi-core.o is added to multiple modules: i2c-apple i2c-pasemi

Rework the driver to have an explicit helper module, letting Kbuild
take care of whether this should be built-in or a loadable driver.

Fixes: 9bc5f4f6 ("i2c: pasemi: Split pci driver to its own file")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Sven Peter <sven@svenpeter.dev>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>