firmware: tegra: Prepare for supporting in-band debugfs

Currently, BPMP debug information is accessible via the Linux debugfs
file-system using a shared-memory scheme. More recent BPMP firmware now
supports accessing the debug information by in-band messaging which does
not require shared-memory. To prepare for adding in-band debugfs support
for the BPMP, move the shared-memory specific initialisation from the
tegra_bpmp_init_debugfs() into a sub-function.

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