ARM: hw_breakpoint: reserve one breakpoint for watchpoint stepping

The current hw_breakpoint code on ARM reserves 1 breakpoint for each
watchpoint that is available. Since debug architectures prior to 7.1
are restricted to 1 watchpoint anyway, only one breakpoint was ever
reserved.

This patch changes the reservation strategy so that a single breakpoint
is reserved, regardless of the number of watchpoints. This is in
preparation for multiple-watchpoint support on debug architectures
from 7.1 onwards.

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