Pic32mx.cfg: Change system clock to 8Mhz after reset-init.

Pic32mx.cfg: Change system clock to 8Mhz after reset-init.

As for openocd 0.6.0-rc2 the function mips32_pracc_fastdata_xfer()
should now work at a scan frequency up to 1200Khz.
Mainly usefull to increase programming speed.

Also verify_image should be slightly faster.

Change-Id: I1e9b2be73690a4597e2f6ba069c1205026850f07
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/805
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>