ARM: hw_breakpoint: add support for multiple watchpoints

ARM debug architecture 7.1 mandates that the DFAR is updated on a
watchpoint debug exception to contain the faulting virtual address
of the memory access. This allows us to determine which watchpoints
have fired and therefore report useful information to userspace.

This patch adds support for using the DFAR in the watchpoint handler,
which allows us to support multiple watchpoints on CPUs implementing
the new debug architecture.

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