examples: Add options to increase frame rate on FVP/AVH

Because the FVP or AVH is an emulation tool running on a PC, the
performance can be very low compared to an FPGA. The full Arm-2D
benchmark example runs at around 1 frame/second. To improve the user
experience, add options
* `ARM2D_EXAMPLE_TINY_MODE` (ON by default) to build a simpler version
of the example.
* `ARM2D_EXAMPLE_MVE` (OFF by default) to toggle the use of MVE which is
very slow to emulate on the FVP/AVH (which the examples run on
initially), although MVE is one of the key features of the Armv8.1-M
architecture and benefits performance on real hardware.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>