ARM: OMAP: Split plat/serial.h for omap1 and omap2+

For omap1, we'll keep mach/serial.h around for 8250.c hardware
workarounds. For omap2+, we no longer need mach/serial.h and
can make it local to mach-omap2.

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