ARM: Add documentation for the qemu-arm board

Add brief documentation for the recently merged qemu-arm board.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>

diff --git a/doc/README.qemu-arm b/doc/README.qemu-arm
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+#
+# Copyright (C) 2017, Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
+#
+# SPDX-License-Identifier:     GPL-2.0+
+#
+
+U-Boot on QEMU's 'virt' machine on ARM
+======================================
+
+QEMU for ARM supports a special 'virt' machine designed for emulation and
+virtualization purposes. This document describes how to run U-Boot under it.
+
+The 'virt' platform provides the following as the basic functionality:
+
+    - A freely configurable amount of CPU cores
+    - U-Boot loaded and executing in the emulated flash at address 0x0
+    - A generated device tree blob placed at the start of RAM
+    - A freely configurable amount of RAM, described by the DTB
+    - A PL011 serial port, discoverable via the DTB
+    - An ARMv7 architected timer
+    - PSCI for rebooting the system
+    - A generic ECAM-based PCI host controller, discoverable via the DTB
+
+Additionally, a number of optional peripherals can be added to the PCI bus.
+
+Building U-Boot
+---------------
+Set the CROSS_COMPILE and ARCH=arm environment variables as usual, and run:
+
+    make qemu_arm_defconfig
+    make
+
+Running U-Boot
+--------------
+The minimal QEMU command line to get U-Boot up and running is:
+
+    qemu-system-arm -machine virt,highmem=off -bios u-boot.bin
+
+The 'highmem=off' parameter to the 'virt' machine is required for PCI to work
+in U-Boot.
+
+Additional peripherals that have been tested to work in both U-Boot and Linux
+can be enabled with the following command line parameters:
+
+- To add a Serial ATA disk via an Intel ICH9 AHCI controller, pass e.g.:
+    -drive if=none,file=disk.img,id=mydisk -device ich9-ahci,id=ahci -device ide-drive,drive=mydisk,bus=ahci.0
+- To add an Intel E1000 network adapter, pass e.g.:
+    -netdev user,id=net0 -device e1000,netdev=net0
+- To add an EHCI-compliant USB host controller, pass e.g.:
+    -device usb-ehci,id=ehci
+- To add a NVMe disk, pass e.g.:
+    -drive if=none,file=disk.img,id=mydisk -device nvme,drive=mydisk,serial=foo
+
+These have been tested in QEMU 2.9.0 but should work in at least 2.5.0 as well.