clk: allow building lan966x as a module

Set the COMMON_CLK_LAN966X option as a tristate and switch from
builtin_platform_driver() to module_platform_driver() to allow building
and using this driver as a module.

Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Link: https://lore.kernel.org/r/20220617103306.489466-1-clement.leger@bootlin.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>