clk: add clk_valid()

clk: add clk_valid()

Add clk_valid() to check for optional clocks are valid.
Call clk_valid() in test/dm/clk.c and add relevant test routine to
sandbox clk tests.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>