ARM: hw_breakpoint: don't advertise reserved breakpoints

To permit handling of watchpoint exceptions without signalling a
debugger, it is necessary to reserve breakpoint registers for in-kernel
use only.

This patch ensures that we record and subtract the number of reserved
breakpoints from the number of usable breakpoint registers that we
advertise to userspace via the ptrace API.

Signed-off-by: Will Deacon <will.deacon@arm.com>