uio: Support 2^MINOR_BITS minors

register_chrdev limits uio devices to 256 minor numbers which causes
problems on one system I have with 384+ uio devices.  So instead set
UIO_MAX_DEVICES to the maximum number of minors and use
alloc_chrdev_region to reserve the uio minors.

The final result is that the code works the same but the uio driver now
supports any minor the idr allocator comes up with.

Signed-off-by: Eric W. Biederman <ebiederm@aristanetworks.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Hans J. Koch <hjk@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>