Add the up_systemreset interface to the samv7 arch. The approach is slightly...

Add the up_systemreset interface to the samv7 arch.  The approach is slightly different in that: 1) It enables ARCH_HAVE_RESET and allows the user to set if, and for how long, to drive External nRST signal. It also does not contain a default board_reset, as that really should be done in the config's src if CONFIG_BOARDCTL_RESET is defined.