firmware: tegra: Add BPMP debugfs support

Tegra power management firmware running on the co-processor (BPMP)
implements a simple pseudo file system akin to debugfs. The file
system can be used for debugging purposes to examine and change the
status of selected resources controlled by the firmware (such as
clocks, resets, voltages, powergates, ...).

Add support to "mirror" the firmware's file system to debugfs. At
boot, query firmware for a list of all possible files and create
corresponding debugfs entries. Read/write of individual files is
implemented by sending a Message ReQuest (MRQ) that passes the full
file path name and data to firmware via DRAM.

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