arm64/debug: Mask off all reserved bits from generated ESR values

There are only 16 comment bits in a BRK instruction, which
correspond to ESR bits 15:0.  Bits 24:16 of the ESR are RES0,
and might have weird meanings in the future.

This code inserts 16 bits of comment in the ESR value instead of
20 (almost certainly a typo in the original code).

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>