net: devlink: add DEVLINK_INFO_VERSION_GENERIC_FW_BOOTLOADER

As discussed in [1], abbreviating the bootloader to "bl" might not be
well understood. Instead, a bootloader technically being a firmware,
name it "fw.bootloader".

Add a new macro to devlink.h to formalize this new info attribute name
and update the documentation.

[1] https://lore.kernel.org/netdev/20221128142723.2f826d20@kernel.org/

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://lore.kernel.org/all/20221130174658.29282-5-mailhol.vincent@wanadoo.fr
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>