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

tcl/board/sheevaplug.cfg

@@ -3,6 +3,8 @@
 source [find interface/ftdi/sheevaplug.cfg]
 source [find target/feroceon.cfg]
 
+adapter_khz 2000
+
 $_TARGETNAME configure \
 	-work-area-phys 0x10000000 \
 	-work-area-size 65536 \