mach-bcm2708: fix mach/debug-macro.S so that early printk works

Not sure what the original code was trying to do as it was completely
wrong on many levels. This patch fixes the macro to return the correct
physical and virtual addresses of the PL011 UART on the RPi.

Note that you need to boot the compressed kernel (zImage) so that the
UART is configured at boot, or your kernel will hang when it tries to
access the UART.

Signed-off-by: Chris Boot <bootc@bootc.net>