cs-2000-cp: keep Reserved bit on each register

Thus CS2000 datasheet is indicating below, this patch
follows it.

WARNING: All "Reserved" registers must maintain their default
         state to ensure proper functional operation.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>