firmware: qcom: scm: Change initcall to subsys

The patch changes the initcall for SCM to use subsys_initcall
instead of arch_initcall.  This corrects the order so that we don't
probe defer when trying to get clks which causes issues later when
the spm driver makes calls to qcom_set_warm_boot_addr().

The order became an issue due to the changes to use arch_initcall_sync
for of_platform_default_populate_init().

Signed-off-by: Andy Gross <andy.gross@linaro.org>