soc: stm32: convert to use DEVICE_DT_GET for clocks

Convert from device_get_binding to DEVICE_DT_GET.  In doing this we
no longer need the label in the devicetree node so we remove that.

Removed all __ASSERT_NO_MSG(clk) since we'll get a build error if
DEVICE_DT_GET cant be satisfied, and the clock control api's will
handle reporting if the device_is_ready.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>