spmi: pmic-arb: instantiate spmi_devices at arch_initcall

The spmi arbiter device spawns spmi_devices which in turn spawn
platform_devices for pmic peripherals.

Move the arbiter's driver init to arch_initcall so that the subsequent
devices it spawns get instantiated earlier.

Change-Id: Ib96937f274745549ee86f758673aab4db97acbaf
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>