armv7a: access monitor registers only with security extensions

armv7a: access monitor registers only with security extensions

Accordingly to ARM DDI 0406C at B1.5, the security extensions for
armv7a are optional extensions and can be detected by reading
ID_PFR1.
The monitor mode is part of the security extensions and the shadow
registers "sp_mon", "lr_mon" and "spsr_mon" are only present with
the security extensions.

Read the register ID_PFR1 during cortex_a examine, determine if
security extension is present and then conditionally enable the
visibility of the monitor mode shadow registers.

Change-Id: Ib4834698659046566f6dc5cd35b44de122dc02e5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5259
Tested-by: jenkins