arm: make it possible to disable HAS_GICV3

arm: make it possible to disable HAS_GICV3

Today it is a silent option. This patch adds a one line description and
makes it optional.

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Julien Grall <julien.grall@arm.com>
CC: George.Dunlap@eu.citrix.com
CC: Ian.Jackson@eu.citrix.com
CC: jbeulich@suse.com
CC: andrew.cooper3@citrix.com
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Changes in v3:
- remove any changes to MEM_ACCESS
- update commit message

Changes in v2:
- make HAS_GICv3 depend on ARM_64
- remove modifications to ARM_HDLCD kconfig, it has been removed