arch-arm: Fix AArch32 branch instructions disassemble

This patch adds the generateDisassembly method for BranchReg, BranchImm
and BranchRegReg Base classes used by AArch32 branch instructions.

Change-Id: I6de015cc213335556d5187df3d4fcd765876262c
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/9503
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>