xen/arm32: head: Introduce print_reg

xen/arm32: head: Introduce print_reg

At the moment, the user should save r14/lr if it cares about it.

Follow-up patches will introduce more use of putn in place where lr
should be preserved.

Furthermore, any user of putn should also move the value to register r0
if it was stored in a different register.

For convenience, a new macro is introduced to print a given register.
The macro will take care for us to move the value to r0 and also
preserve lr.

Lastly the new macro is used to replace all the callsite of putn. This
will simplify rework/review later on.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>