cfg: LPC17xx default to using SYSRESETREQ to reset target

cfg: LPC17xx default to using SYSRESETREQ to reset target

Originally the LPC17xx user guide (UM10360 Rev 2) stated that SYSRESETREQ
was not supported, so this was the default cortex_m reset mode.

Rev 3 of the same user guide states that it is now supported.
This has been verified on a LPC1768 mbed platform, previously I have not
tested this functionality.

Change-Id: I4858248903981a1c93ce75016e67c9e02702fcc5
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1892
Tested-by: jenkins
Reviewed-by: Jörg Fischer <turboj@gmx.de>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>