firmware: tegra: Refactor BPMP driver

Split BPMP driver into common and chip specific parts to facilitate
adding support for previous and future Tegra chips that are using BPMP
as co-processor.

Signed-off-by: Timo Alho <talho@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>