net: wwan: t7xx: Introduce power management

Implements suspend, resumes, freeze, thaw, poweroff, and restore
`dev_pm_ops` callbacks.

From the host point of view, the t7xx driver is one entity. But, the
device has several modules that need to be addressed in different ways
during power management (PM) flows.
The driver uses the term 'PM entities' to refer to the 2 DPMA and
2 CLDMA HW blocks that need to be managed during PM flows.
When a dev_pm_ops function is called, the PM entities list is iterated
and the matching function is called for each entry in the list.

Signed-off-by: Haijun Liu <haijun.liu@mediatek.com>
Signed-off-by: Chandrashekar Devegowda <chandrashekar.devegowda@intel.com>
Co-developed-by: Ricardo Martinez <ricardo.martinez@linux.intel.com>
Signed-off-by: Ricardo Martinez <ricardo.martinez@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>