i2c: pasemi: split driver into two separate modules

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>