block: cgroups, kconfig, build bits for BFQ-v7r2-3.4

Update Kconfig.iosched and do the related Makefile changes to include
kernel configuration options for BFQ. Also add the bfqio controller
to the cgroups subsystem.

Change-Id: I93b0ee8a6e9bfe3cdcf4993b7c083258986ad4af
Signed-off-by: Paolo Valente <paolo.valente@unimore.it>
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>