net: fill in MODULE_DESCRIPTION()s for cpsw-common

W=1 builds now warn if module is built without a MODULE_DESCRIPTION().
Add descriptions to the TI CPSW switch module.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>