arm_opcode: fix encoding of ARMv5 breakpoint instruction

arm_opcode: fix encoding of ARMv5 breakpoint instruction

The encoding of BKPT instruction is 0xE12###7#, where the four '#'
characters should be replaced by the 16 bits immediate value.
The macro uses an incorrect shift amount, thus the immediate value
is not properly coded and bits 20~23 of the opcode could get
corrupted.

Fixed by using the proper shift amount.

Change-Id: I32db8224ab57aad6d3b002f92f9f259056593675
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4854
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>