driver/linuxgpiod: add support for opendrain srst

driver/linuxgpiod: add support for opendrain srst

Some MCUs (e.g. the STM32F3) directly expose the internal reset line to
an external pin. When this signal is driven by a push/pull line, it can
actually be inhibited by the external driver. This results in a setup
where the MCU cannot reset itself, for example, by a watchdog timeout or
a sysreset request. To fix this condition, support for open drain output
on the SRST line is required.

Note that because `reset_config srst_open_drain` is the default, all
users of this adapter will switch over to an open drain output unless
explicitly configured otherwise.

Signed-off-by: Alex Crawford <openocd@code.acrawford.com>
Change-Id: I89b39b03aa03f826ed3c45793412780448940bcc
Reviewed-on: https://review.openocd.org/c/openocd/+/6559
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>