hw/omap.h: Use TARGET_PRIxPHYS to define OMAP_FMT_plx

Use the new TARGET_PRIxPHYS macro to avoid the need to define an
OMAP_FMT_plx macro whose expansion depends directly on
TARGET_PHYS_ADDR_BITS.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>