Audio policy config xsd: add missing gains and address in ports

No nexus/pixel device uses port gains and their configuration were taken
as reference for the xsd creation.

Gains in mixPort and devicePort are supported by the code and use by
oem. As a result the xsd should allow them.

For validation of this path, the xsd was run against the example xml in
the audiopolicy source. Several other misalignment were found. They will
be fix in an other patch.

The address is also an optional field that was forgotten for the same
reason.

Bug: b/63827061
Test: -noout xmllint --schema hardware/interfaces/audio/2.0/config/audio_policy_configuration.xsd frameworks/av/services/audiopolicy/config/audio_policy_configuration.xml
Test: the above command fails for some other xml node unrelated to this bug
Test: this is tracked by b/38184704
Change-Id: I8dae15eb85a6a6d43c87aa747daf92a88d3fdcc0
Signed-off-by: Kevin Rocard <krocard@google.com>