It is not possible to build only the drivers/infiniband/hw/ (or ulp/)
subdirectory with command such as:

$ make ARCH=x86_64 O=./obj-x86_64/ drivers/infiniband/hw/

This fails with following error messages:

make[2]: Nothing to be done for `all'.
make[2]: Nothing to be done for `relocs'.
CHK include/config/kernel.release
Using /home/ydroneaud/src/linux as source for kernel
GEN /home/ydroneaud/src/linux/obj-x86_64/Makefile
CHK include/generated/uapi/linux/version.h
CHK include/generated/utsrelease.h
CALL /home/ydroneaud/src/linux/scripts/checksyscalls.sh
/home/ydroneaud/src/linux/scripts/Makefile.build:44: /home/ydroneaud/src/linux/drivers/infiniband/hw/Makefile: No such file or directory
make[2]: *** No rule to make target `/home/ydroneaud/src/linux/drivers/infiniband/hw/Makefile'. Stop.
make[1]: *** [drivers/infiniband/hw/] Error 2
make: *** [sub-make] Error 2

This patch creates a Makefile in hw/ and ulp/ and moves each
corresponding parts of drivers/infiniband/Makefile in the new
Makefiles.

It should not break build except if some hw/ drivers or ulp/ were
allowed previously to be built while CONFIG_INFINIBAND is set to 'n',
but according to drivers/infiniband/Kconfig, it's not possible. So it
should be safe to apply.

Signed-off-by: Yann Droneaud
Reviewed-by: Bart Van Assche
Signed-off-by: Roland Dreier