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: NIan Munsie <imunsie@au1.ibm.com>
Tested-by: NMichael Neuling <mikey@neuling.org>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>