pic32mx: false pending at low core clock

To show up the fail try to step with the core clock set to 31.25Khz
and with a ftdi/hs adapter or with a wiggler, -not with ft2232-.
The scan frequency should be set to 300Khz or higher, at lower frequency probably will not fail.

The code exits with error because the pracc address is at 0x0.

It also fails when using the "all" register, but in this case the code works without any message because the
pracc address is at 0xff202004 when it fails.

I never saw this fail with the core clock set to 500Khz or higher, but ...

The workaround simply puts a 1 ms delay after the execution of the DERET instruction.

Change-Id: I38e8c01a9c39aedd3282140543b83a0844d8ad29
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/1139
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>