tty: serial: bcm63xx: lower driver dependencies

Hardware supported by bcm63xx is also used by BCM4908 SoCs family that
is ARM64. In future more architectures may need it as well. There is
nothing arch specific breaking compilation so just stick to requiring
COMMON_CLK.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20201125090608.28442-1-zajec5@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>