Xtensa Demos updated and moved to Partner-Supported submodules (FreeRTOS#1281)

Cadence/Xtensa: Move Tensilica demos to Partner-Supported-Demos submodule

Add a README to this deprecated demos directory indicating where to
find the latest Tensilica test code.

Signed-off-by: Ian Thompson <ianst@cadence.com>

Author: ianstcdns