Add the compatible string of Exynos7's PMU as defined in upstream
dt-schema. This also supports derivative PMUs as defined in schema.
There's no additional setup required here, so pmu_init is skipped.
Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org> Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>