xtensa: replace variant/core.h with asm/core.h

Introduce the header arch/xtensa/include/asm/core.h that provides
definitions for XCHAL macros missing in older xtensa releases. Use this
header instead of variant/core.h

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

xtensa: Add xt2000 support files.

The last check-in to support the xt2000 development platform was missing
some files.

Signed-off-by: Chris Zankel <chris@zankel.net>