powerpc: Merge serial.h

[Paul Mackerras]

David Gibson writes:

> This patch merges the ppc32 and (trivial) ppc64 versions of serial.h.
> Mostly this is just an #ifdef merge, with the 32/64 ifdef being folded
> in with the existing ifdefs for various embedded platforms from the
> 32-bit version.  Notable changes:
> 	- We fold ppc32's pc_serial.h into serial.h, because there's
> no clear reason for keeping it separate.
> 	- We abolish the SERIAL_DEV_OFFSET macro; no-one was using it.

For include/asm-powerpc, I would rather decree that serial ports have
to be presented in the device tree, and have an almost empty serial.h,
leaving the ifdef mess in include/asm-ppc for now.

Paul.