sfc: document devlink flash support

Update the information in sfc's devlink documentation including
 support for firmware update with devlink flash.
Also update the help text for CONFIG_SFC_MTD, as it is no longer
 strictly required for firmware updates.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://patch.msgid.link/3476b0ef04a0944f03e0b771ec8ed1a9c70db4dc.1739186253.git.ecree.xilinx@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>