arch-arm: Correct SecureMonitorTrap vals for aarch32

arch-arm: Correct SecureMonitorTrap vals for aarch32

This patch replaces the dummy values which were defined for the
SecureMonitorTrap thus enabling its usage in aarch32 mode.  1) It
changes the vector table offset from 0x14 to 0x4 in compliance with the
armv8 documentation.  2) When trapping in monitor mode for aarch32, the
mon_lr is updated with the pc + a non zero offset (+4/2 depending on the
current instruction set: +4 for A32, +2 for T32).

Change-Id: I01e1e52bf5ecd405e7472e31e01cf9a599153b08
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8041
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>