west: jlink: adding optional flash loader flag required for some boards

With revision 7.70d, Segger changed the command line interface of the
jlink client, requiring some flash specific information, such as
the base address and the type of memory, e.g. QSPI.

This commit adds an optional argument to the West jlink runner,
'--loader' which passes the information to the jlink client.
This change is backward compatible with versions of jlink older
than v7.70d which introduces the CLI change.

Fixes: #50327

Signed-off-by: Yves Vandervennet <yves.vandervennet@nxp.com>