jtagspi/pld: add interface to get support from pld drivers

jtagspi/pld: add interface to get support from pld drivers

Jtagspi is using a proxy bitstream to "connect" JTAG to the
SPI pins. This is not possible with all FPGA vendors/families.
In this cases a dedicated procedure is needed to establish such
a connection.

This patch adds a jtagspi-mode for these cases. It also adds the
needed interfaces to jtagspi and the pld-driver so the driver
can select the mode and provide the necessary procedures.

For the cases where a proxy bitstream is needed, the pld driver
will select the mode and provide instruction code needed in this
case.

Change-Id: I9563f26739589157b39a3664a73d91152cd13f77
Signed-off-by: Daniel Anselmi <danselmi@gmx.ch>
Reviewed-on: https://review.openocd.org/c/openocd/+/7822
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>