boards: lowrisc: adopt new zephyr:board directive and role

This updates the documentation of all the lowRISC boards to use
the new `zephyr:board::` directive.

Signed-off-by: Benjamin Cabé <[email protected]>

Author: kartben

boards/lowrisc/opentitan_earlgrey/doc/index.rst

@@ -1,7 +1,4 @@
-.. _opentitan_earlgrey:
-
-OpenTitan Earl Grey
-###################
+.. zephyr:board:: opentitan_earlgrey
 
 Overview
 ********