apb: rename apb.c to sabre.c

This is the final stage in correcting the naming convention with respect to
sabre, APB and PBM. It is effectively a file rename from apb.c to sabre.c
along with touching up a few constants to remove the remaining references
to APB.

Note that as part of the rename process the configuration variable
CONFIG_PCI_APB is changed to CONFIG_PCI_SABRE.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: Artyom Tarasenko <atar4qemu@gmail.com>