firmware: tegra: Expose tegra_bpmp_mrq_return()

Expose and export the tegra_bpmp_mrq_return() function for use by
drivers outside the core BPMP driver. This function is used to reply to
messages originating from the BPMP, which is required in the thermal
driver.

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