arm64: hw_breakpoint: Allow EL2 breakpoints if running in HYP

With VHE, we place kernel {watch,break}-points at EL2 to get things
like kgdb and "perf -e mem:..." working.

This requires a bit of repainting in the low-level encore/decode,
but is otherwise pretty simple.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>