arm64/debug: Eliminate magic number for size of BRK instruction

The size of an A64 BRK instruction is the same as the size of all other
A64 instructions, because all A64 instructions are the same size.

BREAK_INSTR_SIZE is retained for readibility, but it should not be
an independent constant from AARCH64_INSN_SIZE.

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>