arm_adi_v5: add dap apreg command for AP register read/write

A developer tool: Direct access to AP registers can be useful
for handling vendor specific AP like Freescale Kinetis MDM or Atmel SMAP.

Change-Id: Ie2c7160fc6b2e398513eb23e1e52cbb52b88d9bd
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/2777
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins

doc/openocd.texi

@@ -7836,6 +7836,12 @@ Displays ID register from AP @var{num},
 defaulting to the currently selected AP.
 @end deffn
 
+@deffn Command {dap apreg} ap_num reg [value]
+Displays content of a register @var{reg} from AP @var{ap_num}
+or set a new value @var{value}.
+@var{reg} is byte address of a word register, 0, 4, 8 ... 0xfc.
+@end deffn
+
 @deffn Command {dap apsel} [num]
 Select AP @var{num}, defaulting to 0.
 @end deffn