firmware: turris-mox-rwtm: Add sysfs documentation

Add sysfs ABI documentation for the sysfs files created by the
turris-mox-rwtm driver.

Link: https://lore.kernel.org/r/20190822014318.19478-4-marek.behun@nic.cz


Signed-off-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>