mmc: dw_mmc: lookup for optional biu and ciu clocks

Some platforms allow for clock gating and control of bus interface unit
clock and card interface unit clock. Add support for clock lookup of
optional biu and ciu clocks for clock gating and clock speed
determination.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>