drivers: net: cpsw: use pinctrl PM helpers

This utilize the new pinctrl core PM helpers to transition
the driver to "default" and "sleep" states.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>