microblaze/mm/highmem: Add dropped #ifdef back

The conversion to generic kmap atomic broke microblaze by removing the
build fail.

Add it back.

Fixes: 7ac1b26b ("microblaze/mm/highmem: Switch to generic kmap atomic")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Michal Simek <monstr@monstr.eu>