Mark CRIS architecture as deprecated

The CRIS architecture support in Buildroot hasn't been updated since a
long time. Even a toolchain with recent kernel headers does not build
due to missing patches.

Moreover, the CRIS architecture has been discontinued by Axis, as
visible at http://www.axis.com/products/dev/index.htm

. We will remove
it from Buildroot at the next release cycle.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>