docs: update physical memory documentation by adding N_GENERIC_INITIATOR to enum node_states

While reading physical_memory.rst, I noticed that N_GENERIC_INITIATOR has not been update
from the node_states list, even though it's already added in commit 894c26a1c274b8eafbb4b1dad67e70e51a106061.

Signed-off-by: Kim Tae Hyun <[e-mail]>
Acked-by: Mike Rapoport (Microsoft) <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/mm/physical_memory.rst

···
   The node has memory(regular, high, movable)
 ``N_CPU``
   The node has one or more CPUs
+``N_GENERIC_INITIATOR``
+  The node has one or more Generic Initiators
 
 For each node that has a property described above, the bit corresponding to the
 node ID in the ``node_states[<property>]`` bitmask is set.