firmware: tegra: Add stubs when BPMP not enabled

Add static inline stubs to bpmp.h when CONFIG_BPMP is not enabled.
This allows building BPMP-related drivers with COMPILE_TEST.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>