tpm: Rename tpm.c to tpm-interface.c

This is preparation for making the tpm module multi-file. kbuild does
not like having a .c file with the same name as a module. We wish to
keep the tpm module name so that userspace doesn't see this change.

tpm-interface.c is chosen because the next several commits in the series
migrate items into tpm-sysfs.c, tpm-dev.c and tpm-class.c. All that will
be left is tpm command processing and interfacing code.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>