linux-firmware: split out more firmware pieces

linux-firmware: split out more firmware pieces

Split _most_ of the rtl_nic drivers to a subpackage. Notably this does
not include rtl8168 as this is already split into a separate package.
These drivers are fairly common so this lets people install the essential
firmware easier.

Also split out some large firmware collections into subpackages:
- Mellanox (82M)
- Marvell Prestera (72M)
- QLogic 4xxxx (21M)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>