yang: mark a couple of prefix-list/access-list leafs as mandatory

yang: mark a couple of prefix-list/access-list leafs as mandatory

The code assumes that these leafs always exist when their complementary
leafs exist. For example, when processing `ipv4-exact-match`, the code
always expects to have access to `ipv4-prefix`. If those leafs are not
provided, code crashes. It doesn't happen when using the CLI because it
always does the right thing, but it can happen when using other
frontends.

Also fix incorrect description for prefix-list sequence leaf.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>