arm64: dts: pm8916: Mark the s2 regulator as always-on

The S2 regulator is used by the CPU. If the cpufreq driver is
disabled or has not probed yet, then this regulator will be
marked as unused and disabled. If this happens, the board will
not boot completely and the last message on the console would be
s2: disabling

Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>