arm/virt: place power button pin number on a define

arm/virt: place power button pin number on a define

Having magic numbers inside the code is not a good idea, as it
is error-prone. So, instead, create a macro with the number
definition.

Link: https://lore.kernel.org/qemu-devel/CAFEAcA-PYnZ-32MRX+PgvzhnoAV80zBKMYg61j2f=oHaGfwSsg@mail.gmail.com/
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-id: ef0e7f5fca6cd94eda415ecee670c3028c671b74.1723121692.git.mchehab+huawei@kernel.org
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>