Add network index mask support in NPU driver

To be able to validate if a network index is supported, the NPU driver
now supports getting a mask of the supported network indexes from the
subsystem.

The network mask has also been made available for the user in the device
capabilities.

Change-Id: I09d8d055748dd584f4e8f7655499d7ab1c7563aa
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>