enetc: add suspend/resume function to manage pm in ethernet driver

When running suspend/resume test on nxp ls1028 platform, there is below
warning trace
Call trace:
 mdio_bus_phy_resume+0xa8/0x128
 dpm_run_callback+0x4c/0x184
 device_resume+0x9c/0x1f0
 dpm_resume+0x114/0x340
 dpm_resume_end+0x24/0x3c
 suspend_devices_and_enter+0x1e0/0x810
 pm_suspend+0x288/0x330
 ......
 el0t_64_sync+0x1a0/0x1a4
---[ end trace e8da30565a047649 ]---
Because there is a commit 47ac7b2f("net: phy: Warn about incorrect
mdio_bus_phy_resume() state") to check phy state when entering resume
phase. This is only a surface phenomenon, the root cause is that enetc driver
can not co-operation with common phy code properly. According to
commit fba863b8

("net: phy: make PHY PM ops a no-op if MAC driver
manages PHY PM "). enetc also can manages PHY PM by itself. So, add
suspend/resume function to manage both mac and phy PM.

Signed-off-by: Meng Li <Meng.Li@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>