xtensa: Added Cadence CSP kernel configuration for Xtensa

Added defconfig, device tree and Xtensa variant header files for the
Cadence Configurable System Platform "xt_lnx" processor configuration.

Signed-off-by: Scott Telford <stelford@cadence.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>