tcl/board/sheevaplug: add adapter_khz setting

This combination is known to work properly with 2MHz JTAG clock.

Change-Id: Ie5ec3d3b415efbb13faee7d34e0c7f862b78350c
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2266
Tested-by: jenkins