Add Linux SPI device SWD adapter support

To alleviate the need to bitbang SWD, I've written a SWD SPI
implementation. This code is inspired by the work of luppy@appkaki.com
as shown at github.com/lupyuen/openocd-spi but with the desire to be
more generic. This implementation makes use of the more common 4 wire
SPI port using full duplex transfers to be able to capture the SWD ACK
bits when a SWD TX operation is in progress.

TEST:
  Connects successfully with the following combinations:
  Hosts:
    Raspberry Pi 4B
    Unnamed Qualcomm SoC with QUPv3 based SPI port
  Targets:
    Raspberry Pi 2040
    Nordic nRF52840
    NXP RT500

Change-Id: Ic2f38a1806085d527e6f999a3d15aea6f32d1019
Signed-off-by: Richard Pasek <rpasek@google.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8645
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: zapb <dev@zapb.de>
Tested-by: jenkins

configure.ac

@@ -163,6 +163,9 @@ m4_define([PCIE_ADAPTERS],
 m4_define([SERIAL_PORT_ADAPTERS],
 	[[[buspirate], [Bus Pirate], [BUS_PIRATE]]])
 
+m4_define([LINUXSPIDEV_ADAPTER],
+	[[[linuxspidev], [Linux spidev driver], [LINUXSPIDEV]]])
+
 # The word 'Adapter' in "Dummy Adapter" below must begin with a capital letter
 # because there is an M4 macro called 'adapter'.
 m4_define([DUMMY_ADAPTER],
@@ -289,6 +292,7 @@ AC_ARG_ADAPTERS([
   LIBFTDI_ADAPTERS,
   LIBFTDI_USB1_ADAPTERS,
   LIBGPIOD_ADAPTERS,
+  LINUXSPIDEV_ADAPTER,
   SERIAL_PORT_ADAPTERS,
   DUMMY_ADAPTER,
   PCIE_ADAPTERS,
@@ -726,6 +730,7 @@ PROCESS_ADAPTERS([LIBJAYLINK_ADAPTERS], ["x$use_internal_libjaylink" = "xyes" -o
 PROCESS_ADAPTERS([PCIE_ADAPTERS], ["x$is_linux" = "xyes"], [Linux build])
 PROCESS_ADAPTERS([SERIAL_PORT_ADAPTERS], ["x$can_build_buspirate" = "xyes"],
                                          [internal error: validation should happen beforehand])
+PROCESS_ADAPTERS([LINUXSPIDEV_ADAPTER], ["x$is_linux" = "xyes"], [Linux spidev])
 PROCESS_ADAPTERS([DUMMY_ADAPTER], [true], [unused])
 
 AS_IF([test "x$enable_linuxgpiod" != "xno"], [
@@ -875,6 +880,7 @@ m4_foreach([adapter], [USB1_ADAPTERS,
 	LIBFTDI_USB1_ADAPTERS,
 	LIBGPIOD_ADAPTERS,
 	LIBJAYLINK_ADAPTERS, PCIE_ADAPTERS, SERIAL_PORT_ADAPTERS,
+	LINUXSPIDEV_ADAPTER,
 	DUMMY_ADAPTER,
 	OPTIONAL_LIBRARIES,
 	COVERAGE],