net: fill in MODULE_DESCRIPTION()s for litex

W=1 builds now warn if module is built without a MODULE_DESCRIPTION().
Add descriptions to the LiteX Liteeth Ethernet device.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Gabriel Somlo <gsomlo@gmail.com>
Link: https://lore.kernel.org/r/20240123190332.677489-10-leitao@debian.org


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