clk: atlas7: export mux clks so that consumers can get them

this patch makes mux clks can be referenced by device drivers.

Signed-off-by: Guo Zeng <guo.zeng@csr.com>
Signed-off-by: Barry Song <Barry.Song@csr.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>