docs/system: arm: Add nRF boards description

This adds the target guide for BBC Micro:bit.

Information is taken from https://wiki.qemu.org/Features/MicroBit
and from hw/arm/nrf51_soc.c.

Signed-off-by: Alexandre Iooss <erdnaxe@crans.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Message-id: 20210621075625.540471-1-erdnaxe@crans.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

MAINTAINERS

@@ -1031,6 +1031,7 @@ F: hw/*/microbit*.c
 F: include/hw/*/nrf51*.h
 F: include/hw/*/microbit*.h
 F: tests/qtest/microbit-test.c
+F: docs/system/arm/nrf.rst
 
 AVR Machines
 -------------

docs/system/arm/nrf.rst

@@ -0,0 +1,51 @@
+Nordic nRF boards (``microbit``)
+================================
+
+The `Nordic nRF`_ chips are a family of ARM-based System-on-Chip that
+are designed to be used for low-power and short-range wireless solutions.
+
+.. _Nordic nRF: https://www.nordicsemi.com/Products
+
+The nRF51 series is the first series for short range wireless applications.
+It is superseded by the nRF52 series.
+The following machines are based on this chip :
+
+- ``microbit``       BBC micro:bit board with nRF51822 SoC
+
+There are other series such as nRF52, nRF53 and nRF91 which are currently not
+supported by QEMU.
+
+Supported devices
+-----------------
+
+ * ARM Cortex-M0 (ARMv6-M)
+ * Serial ports (UART)
+ * Clock controller
+ * Timers
+ * Random Number Generator (RNG)
+ * GPIO controller
+ * NVMC
+ * SWI
+
+Missing devices
+---------------
+
+ * Watchdog
+ * Real-Time Clock (RTC) controller
+ * TWI (i2c)
+ * SPI controller
+ * Analog to Digital Converter (ADC)
+ * Quadrature decoder
+ * Radio
+
+Boot options
+------------
+
+The Micro:bit machine can be started using the ``-device`` option to load a
+firmware in `ihex format`_. Example:
+
+.. _ihex format: https://en.wikipedia.org/wiki/Intel_HEX
+
+.. code-block:: bash
+
+  $ qemu-system-arm -M microbit -device loader,file=test.hex

docs/system/target-arm.rst

@@ -87,6 +87,7 @@ undocumented; you can get a complete list by running
    arm/digic
    arm/musicpal
    arm/gumstix
+   arm/nrf
    arm/nseries
    arm/nuvoton
    arm/orangepi