arm64: Invoke the of_platform_populate() at arch_initcall() level

The of_platform_populate() is currently invoked at device_initcall()
level. There are however drivers that use platform_driver_probe()
directly and they need the devices to be populated. This patch makes the
of_platform_populate() and arch_initcall().

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: Benoit Lecardonnel <Benoit.Lecardonnel@synopsys.com>
Tested-by: Benoit Lecardonnel <Benoit.Lecardonnel@synopsys.com>