drivers: soc: sdx_ext_ipc: Make gpio DT entries optional

To reuse the driver for multiple configurations where all the gpios
are not mandatory. Make gpios optional to get probe successful.

Change-Id: Iacfe4a09d55caa1622fdbc2470c693a8610ec1de
Signed-off-by: Arun KS <arunks@codeaurora.org>