gpio: Add bmc-secure-boot

This is to be used on P10 BMC systems that have a jumper that can be
used to disable/enable the enforcement of secure boot policy.

Change-Id: Ia35f85441869eb3a3e384fcd4eb2b71625f86c2b
Signed-off-by: Joel Stanley <joel@jms.id.au>

designs/device-tree-gpio-naming.md

@@ -95,6 +95,16 @@ Defined:
 - ...
 - presence-ps`<N>`
 
+### Secure Boot
+
+#### bmc-secure-boot
+
+Input pin that indicates that the BMC is configured to boot with security
+protections enforced.
+
+Pulled up by default (secure). Placing the jumper will pull the pin down
+(bypass security).
+
 ### Special
 These are special case and/or grandfathered in pin names.