jtag/drivers: bcm2835gpio: implement memory barriers when bitbashing

jtag/drivers: bcm2835gpio: implement memory barriers when bitbashing

This GPIO driver is common to SoCs that have in-order ARM cores
(BCM2835) as well as superscalar (BCM2836-7) and speculative
out-of-order cores (BCM2711).

For BCM2837 and BCM2711, the processor can dual-issue stores and
is free to merge writes to peripheral memory for pages mapped
MT_NORMAL_NC, which is the default provided by /dev/[gpio]mem.

This can cause glitches (or missing edges) on GPIO pins when
toggled with no delay, as pipelined writes to the same address
can get arbitrarily squelched.

To prevent this happening, make sure the preceding write ops are
flushed outside the shareable domain by using a memory barrier.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>
Change-Id: I8805cc0911667bcb9b7f4ca340d7f4f1cb25d096
Reviewed-on: https://review.openocd.org/c/openocd/+/7258
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>