hw/gpio/nrf51_gpio: Add nRF51 GPIO peripheral

hw/gpio/nrf51_gpio: Add nRF51 GPIO peripheral

This adds a model of the nRF51 GPIO peripheral.

Reference Manual: http://infocenter.nordicsemi.com/pdf/nRF51_RM_v3.0.pdf

The nRF51 series microcontrollers support up to 32 GPIO pins in various configurations.
The pins can be used as input pins with pull-ups or pull-down.
Furthermore, three different output driver modes per level are
available (disconnected, standard, high-current).

The GPIO-Peripheral has a mechanism for detecting level changes which is
not featured in this model.

Signed-off-by: Steffen Görtz <contrib@steffen-goertz.de>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20190103091119.9367-6-stefanha@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>