clk: analogbits: Allow building the library as a module

This library is only used by the SiFive PRCI driver. When that driver is
built as a module, it makes sense to build this library as a module too.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Link: https://lore.kernel.org/r/20230725004248.381868-1-samuel.holland@sifive.com


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