hw/timer: Compile devices not target-dependent as common object

hw/timer: Compile devices not target-dependent as common object

All these devices do not contain any target-specific. While most
of them are arch-specific, they are shared between different
targets of the same arch family (ARM and AArch64, MIPS32/MIPS64,
endianess, ...).
Put them into common-obj-y to compile them once for all targets.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191003230404.19384-2-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>