XO is onchip buffer clock to generate 19.2MHz.

This patch adds support to 5 XO buffer clocks found on PMIC8921,
these buffer clocks can be controlled from external pin or in
manual mode.
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>