[ARM] 4391/1: make at91 debug unit support optional

The AT91x40 family doesn't have the debug unit like its bigger brothers.
But it does have the ID and extension registers (with the bit meanings
the same). Reorganize at91_dbgu.h to cater for this.

This also affects the load uncompressor, since it outputs to the
debug port.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>