mailbox: ARM_MHU_V3 should depend on ARM64

The ARM MHUv3 controller is only present on ARM64 SoCs.  Hence add a
dependency on ARM64, to prevent asking the user about this driver when
configuring a kernel for a different architecture than ARM64.

Fixes: ca1a8680 ("mailbox: arm_mhuv3: Add driver")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>