pic32mx: 0 wait state option

By default pic32mx starts after any reset with 1 wait state for RAM access/exec.
It can be changed to 0 wait states by clearing the BMXWSDRM bit (bit 6) in BMXCON register.
With 0 wait states near doubles the execution speed. CRC check sum can be done much faster
increasing verify_image speed. Fast data transfer also works with a bit higher scan rate, up to 1500 Khz.
This option can be set at any time with
mww 0xbf882004 0x40
or cleared with
mww 0xbf882008 0x40.
Some numbers for FTDI/HS with current devel code and a elf file:

Core clock / wait states	             verify_image speed
------------------------------------|------------------------------

        4 Mhz    /    1                            21 KiB/s
        4 Mhz    /    0                            36 KiB/s
        8 Mhz    /    1                            37 KiB/s
        8 Mhz    /    0                            57 KiB/s

Change-Id: I4092ad0f3753f72f77108718d0ed3a3ab84e3b23
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/1141
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Xiaofan <xiaofanc@gmail.com>