bsp: Initialize protobuf interface after capsule update provider

Initializing protobuf interface before capsule update provider changes
the capsule update provider ID that is hard-coded by u-boot and breaks
the capsule update mechanism in corstone1000. This moves the
initialization of protobuf interface at end of se-proxy setup phase,
and fixes capsule update mechanism without any change required in
u-boot.

Changelog: bug
Signed-off-by: Emekcan Aras <Emekcan.Aras@arm.com>
Signed-off-by: Adam Johnston <adam.johnston@arm.com>