MIPS: Use GENERIC_IOMAP

MIPS: Use GENERIC_IOMAP

MIPS has a copy of lib/iomap.c with minor alterations, none of which are
necessary given appropriate definitions of PIO_OFFSET, PIO_MASK &
PIO_RESERVED. Provide such definitions, select GENERIC_IOMAP & remove
arch/mips/lib/iomap.c to cut back on the needless duplication.

The one change this does make is to our mmio_{in,out}s[bwl] functions,
which began to deviate from their generic counterparts with commit
0845bb721ebb ("MIPS: iomap: Use __mem_{read,write}{b,w,l} for MMIO"). I
suspect that this commit was incorrect, and that the SEAD-3 platform
should have instead selected CONFIG_SWAP_IO_SPACE. Since the SEAD-3
platform code is now gone & the board is instead supported by the
generic platform (CONFIG_MIPS_GENERIC) which selects
CONFIG_SWAP_IO_SPACE anyway, this shouldn't be a problem any more.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20342/
Cc: linux-mips@linux-mips.org