ARM: Add a new arch + board for QEMU's 'virt' machine

This board builds an U-Boot binary that is bootable with QEMU's 'virt'
machine on ARM. The minimal QEMU command line is:

    qemu-system-arm -machine virt,highmem=off -bios u-boot.bin

(Note that the 'highmem=off' parameter to the 'virt' machine is required for
PCI to work in U-Boot.) This command line enables the following:
    - u-boot.bin 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, QEMU allows plugging a bunch of useful peripherals to the PCI bus.
The following ones are supported by both U-Boot and Linux:

- 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.:
    -net nic,model=e1000 -net user
- 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

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 5d1ce3e..64e0ee4 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -631,6 +631,14 @@
 	select CPU_V7
 	select BOARD_EARLY_INIT_F
 
+config ARCH_QEMU
+	bool "QEMU Virtual Platform"
+	select CPU_V7
+	select ARCH_SUPPORT_PSCI
+	select DM
+	select DM_SERIAL
+	select OF_CONTROL
+
 config ARCH_RMOBILE
 	bool "Renesas ARM SoCs"
 	select DM
@@ -1171,6 +1179,8 @@
 
 source "arch/arm/mach-meson/Kconfig"
 
+source "arch/arm/mach-qemu/Kconfig"
+
 source "arch/arm/mach-rockchip/Kconfig"
 
 source "arch/arm/mach-s5pc1xx/Kconfig"