arm: Clean up asm/io.h

asm/io.h is the header file containing the central MMIO accessor macros.
Judging by the header and the comments, it was apparently once copied
from the Linux kernel, but has deviated since then *heavily*. There is
absolutely no point in staying close to the original Linux code anymore,
so just remove the old cruft, by:
- removing pointless Linux history
- removing commented code
- removing outdated comments
- removing unused definitions (for mem_isa)

This massively improves the readability of the file.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>