microblaze/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: Michal Simek <monstr@monstr.eu>
Cc: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20201103095857.777445435@linutronix.de