firmware: turris-mox-rwtm: add debugfs documentation

Add debugfs ABI documentation for the ECDSA signatures.
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>

Documentation/ABI/testing/debugfs-turris-mox-rwtm

+What:		/sys/kernel/debug/turris-mox-rwtm/do_sign
+Date:		Jun 2020
+KernelVersion:	5.8
+Contact:	Marek Behún <marek.behun@nic.cz>
+Description:	(W) Message to sign with the ECDSA private key stored in
+		    device's OTP. The message must be exactly 64 bytes (since
+		    this is intended for SHA-512 hashes).
+		(R) The resulting signature, 136 bytes. This contains the R and
+		    S values of the ECDSA signature, both in big-endian format.