powerpc: Update Kconfig + Makefile to prepare for server doorbells

Move the rule to build doorbell support out of the Makefile and into a
new Kconfig boolean that platforms can select.

We will add doorbell support to pseries as well in the next patch.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Tested-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>