ARM: list number of HW breakpoints/watchpoints

ARM: list number of HW breakpoints/watchpoints

When starting up, say how many hardware breakpoints and watchpoints
are available on various targets.

This makes it easier to tell GDB how many of those resources exist.
Its remote protocol currently has no way to ask OpenOCD for that
information, so it must configured by hand (or not at all).

Update the docs to mention this; remove obsolete "don't do this" info.
Presentation of GDB setup information is still a mess, but at least
it calls out the three components that need setup.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>