IB: Allow build of hw/ and ulp/ subdirectories independently

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 <ydroneaud@opteya.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/ulp/Makefile

@@ -0,0 +1,5 @@
+obj-$(CONFIG_INFINIBAND_IPOIB)		+= ipoib/
+obj-$(CONFIG_INFINIBAND_SRP)		+= srp/
+obj-$(CONFIG_INFINIBAND_SRPT)		+= srpt/
+obj-$(CONFIG_INFINIBAND_ISER)		+= iser/
+obj-$(CONFIG_INFINIBAND_ISERT)		+= isert/