arm: Add Nordic Semiconductor nRF51 SoC

arm: Add Nordic Semiconductor nRF51 SoC

The nRF51 is a Cortex-M0 microcontroller with an on-board radio module,
plus other common ARM SoC peripherals.

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

This defines a basic model of the CPU and memory, with no peripherals
implemented at this stage.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Message-id: 20180831220920.27113-3-joel@jms.id.au
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: wrapped a few long lines]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>