xtensa: add flat support

Add the arch-specific header for flat support on xtensa in preparation
for the Xtensa S6000 nommu port.

Signed-off-by: Oskar Schirmer <os@emlix.com>
Signed-off-by: Johannes Weiner <jw@emlix.com>
Signed-off-by: Chris Zankel <chris@zankel.net>