ARM: omap2: remove include/mach/ subdirectory

mach/serial.h is only included from arch/arm/mach-omap2/serial.h,
and the other two files are unused and empty.

Remove the directory by relocating the contents of mach/serial.h
and dropping all other references.

Reviewed-by: Tony Lindgren <tony@atomide.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>