ARM: dts: msm: Make S1 and S2 dummy regulators on fsm9900

On FSM9900 targets, S1 and S2 are fixed and set by the bootloaders.
Apps processor may not modify it. These entries are still needed to
fulfill regulator references in various device drivers.

Change-Id: Ie47510a8e35011202dcc8f5d49c57e415dc5817e
Acked-by: Kaushik Sikdar <ksikdar@qti.qualcomm.com>
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>