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>
Signed-off-by: David Wu <david.wu@rock-chips.com>
Change-Id: I78b1edea1f8ef54d3aa3f7610d39d79dd994d1bf