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

---
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

xen/arch/arm/Kconfig

@@ -12,7 +12,6 @@ config ARM_32
 config ARM_64
 	def_bool y
 	depends on 64BIT
-	select HAS_GICV3
 
 config ARM
 	def_bool y
@@ -41,7 +40,13 @@ config ACPI
 	  an alternative to device tree on ARM64.
 
 config HAS_GICV3
-	bool
+	bool "GICv3 driver"
+	depends on ARM_64
+	default y
+	---help---
+
+	  Driver for the ARM Generic Interrupt Controller v3.
+	  If unsure, say Y
 
 config HAS_ITS
         bool