ARM: OMAP2+: Fix relative includes for serial.h

As discussed on linux-arm-kernel, we want to avoid
relative includes for the arch/arm/*omap* code:

http://www.spinics.net/lists/linux-omap/msg80520.html



Fix serial.h by moving it to mach/serial.h.

Signed-off-by: Tony Lindgren <tony@atomide.com>