[RFC] mips: Enable bulk write optimization for all writes

mips_m4k_bulk_write_memory was only called from target_write_buffer as an
optimization when the word count was large enough.

Remove mips_m4k_bulk_write_memory from the target type, causing the default
implementation to call the regular mips_m4k_write_memory instead.

Perform the dispatch to bulk write in mips_m4k_write_memory, enabling the
optimization for target_write_memory() writes with size 4, in addition to
target_write_buffer() writes.

It also enables making the choice of bulk write vs regular write
specifically for the architecture and not relying on the generic target
code to make a sensible decision.

Change-Id: I295f21a67ceaa195802403f2518ea2e0a025c1c7
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1215
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Tested-by: jenkins