xtensa: Use Kbuild infrastructure to handle asm-generic headers

Use Kbuild infrastructure to handle the asm-generic headers
and remove the wrapper headers that call them.

This only affects headers that do nothing but include the generic
equivalent. It does not touch any header that does a little more.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>