ACPI: Move AMBA bus scan handling into arm64 specific directory

Commit fcea0ccf

 ("ACPI: bus: Consolidate all arm specific
initialisation into acpi_arm_init()") moved all of the ARM-specific
initialization into acpi_arm_init(). However, acpi_amba.c being outside
of drivers/acpi/arm64 got ignored and hence acpi_amba_init() was not
moved into acpi_arm_init().

Move the AMBA platform bus support into arm64 specific folder and make
acpi_amba_init() part of acpi_arm_init().

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>