flash/nor/jtagspi: add JTAGSPI driver

flash/nor/jtagspi: add JTAGSPI driver

Many FPGA board speak JTAG and have a SPI flash for their bitstream
attached to them. The SPI flash is programmed by first uploading a
proxy bitstream to the FPGA that connects the JTAG interface to the
SPI interface if the IR contains a certain USER instruction. Then the
SPI flash can be erase, written, read directly through the JTAG DR.

The JTAG and SPI signaling is compatible. Such a proxy bitstream only
needs to connect TDO-MISO, TDI-MOSI, TCK-CLK, and the activate the
chip select when the IR contains the special instruction and the JTAG
state machine is in the DR-SHIFT state.

Change-Id: Ibc21d793a83b36fa37e2704966aa5c837c4dd0d2
Signed-off-by: Robert Jordens <jordens@gmail.com>
Reviewed-on: http://openocd.zylin.com/2844
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>