xtensa/mm/highmem: Switch to generic kmap atomic

No reason having the same code in every architecture

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20201103095858.311016780@linutronix.de