x86: clean up aperture_64.c

1. use symbolic register names where appropriate.
2. num to bus or slot changing
3. handle for new opteron for bus other than 0

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>