microblaze: Add missing irqflags.h header

Without this header local_save_flags() is not defined.

Signed-off-by: Stefan Asserhall <stefan.asserhall@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>