firmware: tegra: Conditionally support SoC generations

Only include support for Tegra210 and Tegra186 in the BPMP driver if
support for those SoCs was selected. This fixes a build failure seen
on 32-bit ARM allmodconfig builds, but could also happen on 64-bit
ARM builds if either Tegra210 or Tegra186 were not selected.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Timo Alho <talho@nvidia.com>
Tested-by: Timo Alho <talho@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>