ARM: vexpress/mps2: introduce MPS2 platform

The Cortex-M Prototyping System (or V2M-MPS2) is designed for
prototyping and evaluation Cortex-M family of processors including the
latest Cortex-M7

It comes with a range of useful peripherals including 8MB single cycle
SRAM, 16MB PSRAM, Ethernet, QSVGA touch screen panel, 4bit RGB VGA
connector, Audio, SPI and GPIO.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>