target: Add default implementation of bulk_write_memory

target: Add default implementation of bulk_write_memory

Remove dummy implementations from all targets except arm7_9 and mips, which
are the only ones with real implementations. Replace with a single default
implementation simply calling target_write_memory().

Change-Id: I9228104240bc0b50661be20bc7909713ccda2164
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1213
Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>