net: wwan: t7xx: Device deep sleep lock/unlock

Introduce the mechanism to lock/unlock the device 'deep sleep' mode.
When the PCIe link state is L1.2 or L2, the host side still can keep
the device is in D0 state from the host side point of view. At the same
time, if the device's 'deep sleep' mode is unlocked, the device will
go to 'deep sleep' while it is still in D0 state on the host side.

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>