This patch uses the defined fields to describe PWRON STRAPs for
better readability.

Signed-off-by: Hao Wu <wuhaotsh@google.com>
Reviewed-by: Patrick Venture <venture@google.com>
Message-id: 20220411165842.3912945-3-wuhaotsh@google.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>