Attempting to connect to jlink with a JTAG frequency specified in 'options' does not honor the specified frequency, subsequently resulting in inability to connect to targets with limited JTAG frequency

https://github.com/pyocd/pyOCD/blob/5166025ae5da5e093d6cfe2b26cae5e1334476e4/pyocd/probe/jlink_probe.py#L254

In jlink_probe.JLinkProbe.connect(), there is a line that calls jlink.JLink.connect():

`
self._link.connect(device_name)

`
The caller only passes the device name to the callee, leaving the remaining two arguments to default to speed = 'auto' and verbose = False. This even if the application requests a specific JTAG frequency through the 'frequency' option. This causes the driver to attempt JTAG speed auto-detection, despite the fact that the application wants to use a specific frequency.

The problem with this is that, when speed = 'auto', the driver seems to start the auto-detection process with the setting of 4MHz. When that fails, it drops to 2MHz. When that fails, it seems that the driver gives up, eventually resulting in an exception. As a result, any use-case where the driver's internal auto-detect algorithm fails for whatever reason, it is impossible to establish a working connection, even if a known working frequency has been specified by the application.

Looking at other code in the caller, a possible fix could look something like this:

...
if device_name is not None:
    # If speed specified through options...
    if self.session.options.get('frequency') != None:
        # ... use the speed from options:
        speed = self.session.options.get('frequency')
    else:
        # otherwise, fall back to auto-detect:
        speed = 'auto'
    self._link.connect(device_name, speed=speed)
...

An equivalent of the above solution was tested in 0.35.1/0.36.0 and it seemed to resolve the issue.

Start in pyocd/probe/jlink_probe.py at the JLinkProbe.connect() call around line 254, then inspect how the session's frequency option is read. Ensure the requested frequency is passed to the J-Link connection while retaining automatic detection when no frequency is specified. Verify against a target with a limited JTAG frequency, as described in the issue.