Please contact Aruba Support. I don't have a 2540 to test with, but on my 2930F (running version 16.08.0002) the command is there:
sw01(config)# aaa authentication rest login radius
local Use local switch user/password database.
none Do not use backup authentication methods.
authorized Allow access without authentication.
server-group Specify the server group to use.
two-factor-type Use the certificate or public key for the first authentication method and username/password for the second authentication method.
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If you are running recent firmware (like 16.08 or newer) and you don't see the commands and are logged in with full manager rights, please work with TAC to see what is wrong.