Documentation/arm64: Update ARM and arch reference

Documentation/arm64: Update ARM and arch reference

This patch clarifies that both Armv8 and v9 are in scope, not
just Armv8 systems.
Also, ARM is re-written as Arm.

Cc: Jeremy Linton <Jeremy.Linton@arm.com>
Cc: James Morse <James.Morse@arm.com>
Cc: Rob Herring <Rob.Herring@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Hanjun Guo <guohanjun@huawei.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-doc@vger.kernel.org
Cc: linux-acpi@vger.kernel.org
Signed-off-by: Jose Marinho <jose.marinho@arm.com>
Reviewed-by: Samer El-Haj-Mahmoud <Samer.El-Haj-Mahmoud@arm.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Link: https://lore.kernel.org/r/20230606093528.1494344-2-jose.marinho@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>