mfd: qcom-spmi-pmic: instantiate pmic peripherals at arch_initcall

The spmi_devices spawn platform devices for pmic peripherals. Some of these
devices are required early on in the boot process.

Initialize the driver at arch_initcall to avoid unnecessary probe
deferrals.

Change-Id: I38474bedcb284ed5a7df1ec8d26f680787f2c074
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>