mfd: syscon: Add optional clock support

Some system control registers need to be clocked, so the registers can
be accessed. Add an optional clock and attach it to regmap.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>