MIPS: Add and use watch register field definitions

MIPS: Add and use watch register field definitions

The files watch.c and ptrace.c contain various magic masks for
WatchLo/WatchHi register fields. Add some definitions to mipsregs.h for
these registers and make use of them in both watch.c and ptrace.c,
hopefully making them more readable.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12729/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>