arm: use addruart macro to establish debug mappings

Since we can get both physical and virtual addresses from the addruart
macro, we can use this to establish the debug mappings.

In the case of CONFIG_DEBUG_ICEDCC, we don't need any mappings, but
may still need to setup r7 correctly.

Incorporating ASM changes from Nicolas Pitre <npitre@fluxnic.net>.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Tested-by: Kevin Hilman <khilman@deeprootsystems.com>