powerpc/security: Show powerpc_security_features in debugfs

This can be helpful for debugging problems with the security feature
flags, especially on guests where the flags come from the hypervisor
via an hcall and so can't be observed in the device tree.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>