arm/dt msmcopper: Configure modem supply regulator as always-on

PM8841 regulator SMPS 3 supplies the modem VDD_MSS rail.  Since
the modem is not being booted by Linux at this point, there is
currently no consumer driver for S3.  This causes problems when
the modem is booted independently of Linux because Linux will
automatically disable S3 when it detects that it is enabled in
late_init.  Therefore, configure S3 as always-on.

Change-Id: Ieb9aecbc9362e7bee283a47e075534d2929b12d4
Signed-off-by: David Collins <collinsd@codeaurora.org>