arm64: Populate the platform devices

This patch add a device_initcall() to populate the platform devices
(of_default_bus_match_table). This allows SoC implementations that do
not require earlier initcalls to avoid any platform-specific code under
arch/arm64.

GIC and generic timer initialisation is done via FDT and CPU notifiers
independently of the SoC code.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>