Revert "opencv: mark as not available on Xtensa"

Now that xtensa toolchain is able to compile huge functions this
workaround is no longer needed.

This reverts commit d2193250.

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