drivers: soc: sdx_ext_ipc: Add wakeup support from system suspend

Remote SoC needs to be waken up before initiating any transport
layer interface (like USB, PCIe). Add two GPIOs (to the AP and
modem) to handle to and fro of the wakeable interrupts.

Wakeup notifications are initiated by the transport layer.
Register notifiers in this driver to act on the request to wakeup.

Change-Id: I4ee3b2e15bcb7af63d71bac2fb54488808eb0b45
Signed-off-by: Shreyas K K <shrekk@codeaurora.org>