video: tegra: host: Export subdomain registration

video: tegra: host: Export subdomain registration

This far the subdomain registration has been allowed for drivers that
use nvhost_acm framework. However, gk20a handles its own power domain bu
it needs to register it as a subdomain of host1x. This patch exports
necessary functionality in public header.

Bug 200003112

Change-Id: Ie3457345c101329cadac4ef8aa71e448629a419b
Signed-off-by: Arto Merilainen <amerilainen@nvidia.com>
Reviewed-on: http://git-master/r/407542
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>