gic: fix merge error and clean up obsolete code

gic: fix merge error and clean up obsolete code

The redundant code was left in gic_set_affinity when merging code
from upstream.

Remove CONFIG_GIC_SET_MULTIPLE_CPUS from Kconfig and related code
from irq-gic.c. This is obsoleted and not being used any more.

Change-Id: Ib68661f247ebc319626546cd610fb7fc7ec2dfa4
Signed-off-by: Bo Yan <byan@nvidia.com>
Reviewed-on: http://git-master/r/712309
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Prashant Gaikwad <pgaikwad@nvidia.com>