clk: add CS2000 Fractional-N driver

This patch adds CS2000 Fractional-N driver as clock provider.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
[sboyd@codeaurora.org: Fix unsigned checked for < 0 in
cs2000_ratio_get()]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>